My thoughts about the story, perhaps interesting to any future reader trying to decipher the "mysterianism" here, a la these two analyses of sci-fi short stories, or my own attempts at exegeses of videogames like Braid or The Witness.  Consider the following also as a token of thanks for all the enjoyment I've recieved from reading Gwern's various analyses over the years.

• "Clarity didn't work, trying mysterianism" is the title of a short story by Scott Alexander, which feels to me like an analogy that is arguably about AI, at least insofar as it is clearly about optimization and its dangers.
  • But that story of Scott Alexander's isn't actually very mysterious!  It's kind of a sci-fi / horror story (albeit in a fantasy setting) where it's pretty clear what's going on.  Conversely, this story I can hardly make heads or tails of.  It feels like there should be some kind of AI connection (what else is there to be mysterian about?  surely not just wikipedia editing...), but if it's there, I cannot find it.
• As other commenters mention, the story's title is a reference to a 1966 sci-fi novel, which I sadly haven't read.
  • This (and to a lesser extent the "eternal september" joke) seem to pin down the date, meaning that there probably can't be much significance to the choice of september 30th in particular for this story -- the date is probably already "explained away" by Gwern presumably wanting to title the story the same as the 1966 novel.  By contrast, the selection of year, 1939, remains more of a free parameter that we need to explain.
• One clear theme of the story (although one I myself am not much interested in) is a bundle of stuff related to the idea of curation, of writing and editing, summarization and commentary, et cetera.
  • The frame story where we're reading a translation of a review of a... book? i'm not sure exactly... by M. Trente explaining the work of an Institute founded to archive materials related to the Thirtieth.
  • The story feels extremely Borgesian, recalling the impossible literary conciets of "Tlon, Ubquar, Orbit Tertius", "The Library of Babel", and "Pierre Menard, Author of the Quixote".
  • The bits about digitization, Xerox, the internet, seem like winking asides to the fact that this story exists somewhat anachronistically, and perhaps more closely reflects a wikipedia-adjacent form of online scholar / nerd culture.  After all, what else but wikipedia would literally have a page devoted to all the events of September 30th?  Where else but wikipedia would I go to learn about the various events being referenced in the story (the WW2 events, the first televized football game, the birth of Jean-Marie Lehn, and so on.)
  • There are many amusing linguistic jokes, such as the parallelism between the history of the Institute and the history of the two world wars, where the "second war in the world of the Institute" is WW2-themed followed by a cold war of sorts ("...after which the conflict chilled") -- this in particular struck me as very similar to Gwern's meditations on second life sentences.  Or the bit about how the institute hopes that "understanding how the past understood the future can help understand the future of understanding the past".
  • The loving list of obscure real-world institutes that bear some resemblence to the Institute of the story: the Museum of Jurassic Technology, the "Pith" (Pitt?) Rivers Museum, Gravity Research Foundation.  (I'm not sure if the Labyrinth Sodality, Iberian Sundial Society, and the Rekord-Club Gründerzeit are also oblique references to real-world organizations, or are perhaps references to fiction.)
  • The bit about "hapax ephemeron".
  • Perhaps what I like most about this theme-cluster, rather than the absurd scholarly self-reference and bizzare concept of taking as an object of study the cross-section of a single day, nor the paradoxical idea of spending so many days focused on the study of just one day (a la In The Future, Everyone Will Be Famous to Fifteen People), but rather the more familiar impossibility of trying to somehow recapture the subjective nature of experience, the ephemeral freshness and vividness that once belonged to all the moments making up one fall day in 1939, using only the tools of writing and historiography and so forth.
    • That the story ends, after so much obscure academic writing and abstract discussion, with a vividly physical description of a baseball game -- "spitting chaw in the dust" -- "cold, crisp, and fair, a touch of looming winter" -- "the sun over the roofline half past five" -- speaks to this theme, IMO.  The impossibility of trying to conjure back up the vivid freshness of experience via thought and study and analysis, the contrast between the immediacy / immanence of the present versus the mind's tendency to always zip about with innumerable little thoughts of the past and future.
    • See also the preceeding section which starts "The skeptical reader will soon be forced to agree".
    • Note also that the gravitational arc of the baseball that ends the story -- A slow lazy arc, every eye silently following it spell-gaped, past the sun, back down, toward no one but me as I casually hold up my glove for the ball, suddenly knowing it had been pitched to me by God Himself, landing with just the slightest thwack… -- is another analogy for the ephemeral passing of a single day.  It's fast-moving, it lasts only a few seconds, yet the ball is described as if suspended.  Indeed, that moment is indeed suspended, as an image in the memory of the (multiple layers of fiction and frame-story quotation deep) anonymous memoirist of 1969 (coincidentally the year of the institute's founding).
      • Rosier was too young in 1939 to have been the batter in that game.  But he lived in Chicago at that time -- perhaps he was at the game and witnessed that moment, and that crystalline memory is what spurred him to found the Institute?
• But is there a yet deeper theme to the story?  Or (like my disappointing experience with Unsong), is it for the most part just puns and references all the way down (plus of course the stuff I've described above)?
  • Revisit Gwern's analysis of "Suzzane Deluge" for how deep the rabbit hole of obscured short-story themes might potentially go.  (If that's the case here, it is much further than I will be able to plumb.)
• First, some dangling loose ends:
  • Why is the story ostensibly translated from French, about an institute in New Orleans, with assorted joking references to "gallophobia" and so forth?
  • What's going on with the "dream diary results, showing anomalous ‘ψ’ predictions of October 1st"?
  • What's up with the adventurous biography of Vincent Rosier?  What kind of impression am I supposed to take from all the wacky details?  Reproducing below:
    • Birth in rural Ireland, emigration to the City of Wind, Chicago, hardscrabble childhood running with youth gangs,
    • the precocious admission to an anthropology major followed by a contretemps while kayaking Papua New Guinea,
    • on behalf of the OSS, which connections proved useful in post-war import-export ventures (making, and losing, his first fortune when his curious  ‘Rosier Cube’ oranges were popular, and then unpopular, in the West).
      • (this bit about making and losing tons of money running an import-export business selling goods during WW2, is also a plot point in Chapter 24 of the novel Catch-22, IIRC?  Rosier Cube oranges are presumably akin to Japan's "square melons".)
    • He was even trusted with back-channel Soviet negotiations over the (still-classified) Mukbang Incident,
      • (seems like a weird reference to a modern-day phenomenon in a story ostensibly set in 1990?)
    • followed by restless expansion into unrelated ventures including his publishing & chemical supply conglomerate (best known for its deodorant),
      • no idea what this is about
    • shrimp fishing (vertically integrated restaurant chain),
      • is this a reference to the movie Forrest Gump, the actual vertically-integrated seafood company Red Lobster, or something about EA's interest in shrimp welfare?
    • legendary commodities trades (famously breaking the Newcastle union through locally-delivered coal futures),
      • a reference to this perhaps-apocryphal story about 28000 tons of mis-delivered coal.
    • and an ill-fated Texas ostrich farm investment (now a minor tourist attraction).
      • part of an actual '90s trend
    • Eventually he withdrew into Californian venture capital, a seersucker-clad gray eminence among Silicon Valley upstarts, where he amassed the Greco-Roman art collection he is chiefly remembered for.
      • Is there some kind of AI or early-internet connection here?  Are the statues some kind of reference to right-wing "statue in profile pic" culture??  Probably not... more likely it's a reference to the Getty museum in california?
  • More exerpts:
    • If Trente’s exponential bibliometric projections are correct, by “152 AE”, no publication on the 20^th century will fail to mention the 30^th.
      • The idea of "exponential bibliometric projections" in this context is obviously absurd, and is funny.  The triumphalism of this section certaintly feels like a reference to the world-transformation project of "Tlon, Ubquar, Orbit Tertius", as one hackernews commenter notes.  But this (along with the idea that "a Palo Alto recluse has changed the earth" -- many such cases!) also feels like the most direct reference (if indeed it is one) to a potential AI theme, since the initial cross-post reference to "Clarity didn't work, trying mysterianism" -- insofar as it evokes/spoofs exponential projections by the likes of Moravec, Vinge, and Kurzweil.
      • But note that 152 AE is the year 2091, which strikes me as a little late for scaling-pilled AGI timelines.
    • It makes little difference to us, as we go on revising, in our quiet countryside retirement, an encyclopedia of Casares we shall never publish.
      • Beyond the Borges reference, if we are trying to force an AI reading, this perhaps sounds like Gwern's description of his own life (writing mysterian short stories, etc) amidst a world so rapidly changing that our present era is perhaps comparable to none other than late September, 1939.
• But perhaps more importantly:
  • Why is the Institute's motto "Lux in Tenebris Diei Unius"?  Google's AI translates as "Light in the Darkness of a Single Day", and comments "It's a variation of the more common phrase 'Lux in tenebris lucet,' which is particularly significant in Christian theology. "
  • Why 1939?
    • Why was the institute founded?  This question is never answered by the story.  Are the "military historian" or "pacifist" factions closer to the truth?
      • If Rosier was born in 1927 then he would've been 12 years old in 1939, which seems too early for him to have been deeply affected by WW2 (especially considering he was living in Chicago, not anywhere in Europe).
      • But, "Pacifist" arguments aside, the choice of year is a free parameter in Gwern's story and obviously significant in connection with World War 2.  So, I am inclined to think that the Pacifists are wrong, and the focus of the Institute (and Gwern's story) is somehow intimately tied up with WW2 in particular.
    • Other than the fact that the Thirtieth has an Institute and today does not, is there some further sense in which today is less "real" than Sept 30, 1939?  The story says "Here, at the end of history, mankind has been disillusioned of ideology and symmetry, and unable to look forward, looks back."  Today, as both the story and Zarathustra and Fukuyama argued, "is no-one and no-where and no-when; and men cannot live in a utopia."
      • 1939 is thus perhaps more sincere (with its many zealous true believers in ideologies like Communism, Fascism, and Democracy), less ironic or cynical or world-weary, and for all those reasons perhaps more real than today.
      • But they are also of course more naive, ignorant, and fundamentally confused (re: those same zealous true believers in Communism and Fascism, the eagerness to launch into patriotic wars, et cetera).
      • Perhaps this is the essential relationship that the past always has to the future?  Today, 2025, as we stand potentially on the cusp of transformative change driven by AI, and also seemingly in a world of steadily increasing international tensions and war (russia/ukraine, then israel/gaza/iran, india/pakistan, perhaps soon china/taiwan?) will probably seem similarly confused and naive and in-denial-about-what's-obviously-coming from the standpoint of 2076, as 1939 seemed from 1990.
• And finally: why is October the First "too late"?
  • Insofar as it's connected to the 1966 novel: Just skimming the wikipedia page, it's about an Earth that's been jumbled up such that different regions are in different eras (eg, mexico is in the far-future, while greece is in classical times, and France is in 1917).  This, combined with the theme of World War 2, puts me in a mind of alternate-history scenarios.  (But maybe it is just meant to point me towards the jumbled-up nature of extensively studying the Thirtieth in the modern day.)
  • Insofar as it isn't just a reference to the 1966 novel, then we must ask ourselves: October 1st 1939 is too late, for what?
    • Well, obviously it's too late to prevent things that happened on Sept 30th or Oct 1st, namely the surrender of Warsaw to Nazi forces (on the 28th) capture of Modlin Fortress (on the 29th), Hitler's partition of Poland and the establishment of a polish government-in-exile (on the 30th), the entry of Nazis into Warsaw (on Oct 1, starting a period of German occupation of the city until 1945), and this Oct 1 speech by Churchill further committing England to the war and calling up all men age 20-22 for conscription.
    • So, by Sept 30th it's obviously already too late to prevent WW2, since it's already happening.  But perhaps it's not too late to prevent or greatly mitigate the Holocaust (about half of the Jews who died were Poles), wheras Oct 1st somehow would be?
    • Or conversely, perhaps on Sept 30 it's not too late to hope for a mitigated World War, smaller in scope than the actual World War we got?  (ie, perhaps Germany invades Poland and France, but Britain looks the other way and decides to make a "separate peace" with Germany, Hitler never makes the mistake of invading the Soviet Union, and America never gets involved?)  Here I'm just going off Churchill's Oct 1 speech.
    • Both of these scenarios seem pretty ridiculous -- the idea that on Sept 30 the holocaust or the severity of WW2 could totally have been prevented, but October the first is too late, is absurd.  Surely there's just not that much variance from day to day!
    • If we wanted to think more rigorously, we might say that the chances of the Holocaust happening vs not happening (or perhaps this should be graded on a sliding scale of total deaths, rather than being considered as a binary event) are initially pretty low -- if you're re-rolling history starting from the year 1900, perhaps something a Holocaust would only happen 1 in 100 times, or 1% odds?  But of course, if you re-roll history starting in June 1941, then chances are almost 100%, since it's basically already happening.  What about an intermediate year, like 1935?  It must have had some intermediate probability.  So, looking back on history, you can imagine this phantom probability rising higher and lower each year -- indeed, each day -- almost like a stock price or prediction-market price.  Some days (like various days on the timeline of Hitler's rise to power) the odds would rise dramatically, other days the odds would fall.  What was the single most important day, when the probability changed the most?  Is that necessarily the same day as the "hingiest" day, the most influential day for that outcome?  (I don't think it necessarily is?  But I'm confused about this.)
    • And for a more emotional, subjective judgement -- at what point does one say that an event could still be prevented?  At 50/50, it could go either way.  At 80/20, the situation is looking grim, but there is still hope.  At 95/5, the outcome is nearly certain -- but it's not crazy to nevertheless hope that things will change, since after all outcomes with 5% probability do occur 5% of the time.  What about 99/1?  At 99.99/0.01, technically you are still somewhere on the logistic success curve, but from a psychological perspective surely one must round this off to zero, and say that all hope is lost, the outcome is virtually certain.  Perhaps in the world of the story, Sept 30 1939 is the moment where the last significant shred of hope disappears, and the probability (either of the Holocaust, or of a smaller-in-scope WW2, or whatever) tragically jumps from something like 98% to 99.95%.
    • All of the above is perhaps true, and yet the absurdity of trying to pin things down to the individual day remains.  The individual days would surely look pretty insignificant even if we knew the probabilities, and of course we don't know the probabilities.  And for any prospective rather than retrospective scenario (like "will there be a nuclear war in the next 50 years"), we don't even really know where to look, what sub-questions to ask, et cetera.
  • This gets back to my idea that "Today, 2025, as we stand potentially on the cusp of transformative change driven by AI, and also seemingly in a world of steadily increasing international tensions and war, will probably seem similarly confused and naive and in-denial-about-what's-obviously-coming from the standpoint of 2076, as 1939 seemed from 1990."
    • Maybe what really makes Oct 1 1939 "too late" isn't that the objective probabilities (in some omniscient god's-eye-view) mean it's too late to avert some actual event, but rather that Sept 30, 1939 was the last possible date one could be in denial about it -- that one could still /hope/ for the war to be short and largely harmless, rather than vast and world-wrecking.  By Oct 1, the terrible truth would be all too clear.  Of course in the real world, many would continue to hope against all hope, but perhaps Oct 1 was in some sense when it was finally over in the minds of all informed observers -- there is going to be a second world war.
    • So why would you establish an Institute to study that exact moment in time?  Surely because you worry that the present day is in a similar situation -- a situation where we don't know what's coming, even though we SHOULD know what's coming.  In retrospect it will seem so obvious, and we will seem to have our heads in the sand, blind men grasping the elephant, et cetera.  "Lux in Tenebris Diei Unius" -- light out of the darkness of a single day.  Maybe if we study September 1939, Rosier is saying, we can learn how to avoid their failure, and see the threat that's coming to us from a direction we don't presently even know how to look in, and thereby achieve some kind of brighter future.

(Seeing Gwern publish something titled "October The First Is Too Late" was as alarming as a dozen Sam Altman vaguetweets.)