To add to this - there's also some sampling bias going on. As the club events opened up, a lot of visitors will be going to get tested regularly (every week, maybe even 2 times per weekend). After the peak day, when events were closed again, they don't get tested for entry proof anymore, so that also lowers positive test counts.
Though I think the general trend holds despite this.
The big mystery remains why Delta suddenly peaked and turned around, first in India, and now in the UK and the Netherlands. These turnarounds are excellent news, and I presume we will see a similar turnaround at a similar point, but what’s causing them to happen so quickly? I don’t know.
Regarding the covid situation in the Netherlands, it was a bit of a perfect storm of many things coming together.- Dance clubs, events and such opened up with few restrictions - primarily entry proof for vaccination/false-tests- Getting a single jab of J&J would instantly count as entry proof, and was heavily promoted for youths - it was also promoted to take it the same day / day before going clubbing as an alternative to testing.- Delta variant appeared- There was some fraud with the QR-code app for entry-proofs, unsure about how prevalent this was.- Opening up of clubbing and such happened *before* it was even possibly for any substantial part of the 15-30 year olds to have gotten the vaccine the "regular way" at time.
Closing clubs has stopped the wave, and now a month later the majority of youths are vaccinated. And clubs are still closed. Hospital/IC admissions are now starting to go up as the initial wave has spread to mostly leftover unvaccinated 50+ year olds (I'd imagine (grand)parents of youth that went clubbing).
What follows is an outline of what happened time by time and some sources.
General positive test timing data: - Lowest point around 27-29th June- Big inflection point up around 5th of July- Peak already at 10th of JulyA strong majority of positive tests was in ages 10-29, of which most again were in range 20-29.
Timeline of events leading up to the peakLate June in general - Ages 20-29 that plan for the "regular" vaccine (both the RNA ones; pfizer/moderna) can start getting their first jab. This does not count as being vaccinated for things where you need vaccination-proof / false-test-proof. The second jab is late July to early August.18th June -  Announcement that by the 26th of June nearly everything is open; events and such that can't be done at 1.5 meters will require vaccination/false-test proofs.23th June -  200K J&J vaccine jabs are available for taking; Youth is encouraged to get it with the slogan "Dansen met Janssen" - "Dancing with J&J". This vaccine only requires one jab so the idea was that it would be easier to get a high vaccination rate under youths using it; you only have to go out of your way once and you're done. A quick easy way to be "covid proof" for when dance clubs, festivals and holidays would open up in the weeks after it, without having to wait 6 weeks or such for a 2nd jab before it officially counts. Phone lines were absolutely swamped by demand. Would save a lot of testing for people who want to go out clubbing every weekend.24th June -  CoronaCheck app updated; being vaccinated counts via this app as entry proof for events. It counts immediately after getting the vaccine, no delay.(weekend 1; 26th, 27th of June)26th June -  175K extra J&J vaccines. Everything opens up. About the lowest point in positive test cases.26th June -  At one club that was open on this very first "freedom night", 195/600 youths have tested positive in the following days.28th June -  Reports on "CoronaCheck fraud"; there's a 'print' functionality for the QR-code, which makes one that doesn't time out after a few minutes. Send it to all your mates, they show it at the door. Many clubs aren't going to do the manual secondary ID card check to see if it matches, as that's frustrating customers and it already shows as "safe".(weekend 2; 3rd, 4th of July)6th July -  A mistake was corrected - Being vaccinated doesn't count as entry proof for the first 2 weeks after vaccination.10th July - Peak day -  New covid rules; Dance/night clubs closed, live entertainment in catering banned, large multi-day festivals get cancelled; work at home if possible.
Vaccination related data; Official sources say that for 81% of positive tests between the 1st & 27th of July the vaccination status was known. Of those with known status, 11% was fully vaccinated, 16% partially, 73% not. Questioning of the majority of hospitals by a group of doctors got some results on vaccination status for covid hospital patients in the week of 12-18th July. About 75% was not vaccinated. About another 5% had a known problem that would reduce vaccination efficiency. People with "at least one jab", per age group: 20-35 is about 60%, 35-45 about 75%, 45-55 about 80%, 55+ is about 90%
Sources Official dutch covid dashboard; https://coronadashboard.rijksoverheid.nl/landelijk/positief-geteste-mensen Official gov site https://www.government.nl/latest/news/2021/06/18/netherlands-to-take-big-step-in-relaxing-measures-almost-everything-allowed-with-1-5-metre-distancing Dutch public broadcasting news https://nos.nl/collectie/13850/artikel/2386354-een-prik-honderden-keren-bellen-ggd-overweldigd-door-belangstelling-janssen-vaccin Official gov site https://www.rijksoverheid.nl/onderwerpen/coronavirus-covid-19/nieuws/2021/06/24/nu-ook-vaccinatiebewijzen-in-coronacheck-app https://www.rijksoverheid.nl/onderwerpen/coronavirus-covid-19/nieuws/2021/06/26/175.000-extra-janssen-vaccins-voor-keuzelijn https://www.rijksoverheid.nl/onderwerpen/coronavirus-covid-19/nieuws/2021/07/06/vaccinatiebewijs-wordt-twee-weken-na-volledige-vaccinatie-verstrekt https://www.rijksoverheid.nl/onderwerpen/coronavirus-covid-19/nieuws/2021/07/09/snelle-toename-besmettingen-dwingt-tot-maatregelen-in-de-zomer random news site; https://nieuws.nl/algemeen/20210729/superspreader-veroorzaakte-corona-uitbraak-in-club-aspen-valley/ random news site; https://www.volkskrant.nl/nieuws-achtergrond/gesjoemel-met-coronacheck-is-niet-gewoon-even-screenshotje-delen~b1419271/ Dutch CDC equivalent, official https://www.rivm.nl/nieuws/meer-ziekenhuisopnames-daling-aantal-positieve-coronatesten Public broadcasting news https://nos.nl/artikel/2391137-veruit-de-meeste-covidpatienten-in-ziekenhuis-zijn-niet-gevaccineerd https://www.rivm.nl/covid-19-vaccinatie/cijfers-vaccinatieprogramma you can get a table from the graph at the bottom