I assume @Samwilson added an all-day event on the 15th that because of his timezone is displaying as the 14th to most of us? Is that right?
Yep, exactly that! Which is odd, because most calendars do ‘all day’ events by date rather than a 24 hour period in a particular timezone. And interestingly, when setting up an all day even there is no timezone selection box.
Really, Wikipedia Day runs from 00:00 in UTC-12 to 24:00 in UTC+14 (i.e. 50 hours).
This had been brought up previously in a slightly different context Showing full day events in local time may lead to confusing results. I’ve just filed a task for it (T240293) and will likely flag this issue for fixing along with a batch of other minor fixes to the calendar.