April Fool's Day is an annual event every year on April 1, and is celebrated by practical jokes or humorous content (such as the posts under this tag). The event can be thought of as "Notice Your Surprise" day.
Special rule for this tag: Please don't add entries to this tag during April Fool's, to not spoil all the surprise as soon as people see the top of the post, but feel free to add any old April Fool's jokes starting on the 2nd.