Around 1599, William Shakespeare famously wrote "Beware the Ides of March" in the play Julius Caesar. And for many years it was just something that was bad for Caesar, who was assassinated by Marcus Junius Brutus the Younger in the senate on that date, which corresponds to March 15, in the year 44 B.C.E.
But from 1918 to 1954 everyone was wary of it, because March 15 used to be tax day.
After the 16th amendment was passed in 1913 allowing the government to collect income taxes, tax day was briefly March 1 before moving in 1918 to March 15th, where it sat for 36 years. In 1954 the IRS reorganized and moved it to its current date of April 15, a shift that wasn't really explained at the time. In 2002, the IRS told Fortune that it was to "spread out the peak workload," but another tax expert, professor Ed McCaffery of USC, explained that as the tax covered more of the middle class, the IRS needed to issue more refunds. Pushing back the date allowed the Feds to hang on your money longer, McCaffery said.
And what is (not are) the Ides of March exactly? According to the dictionary, "ides" was traditionally the midpoint of a calendar month, usually the 13th or the 15th.
If you want to get into the weeds, the 355-day Roman calendar didn't number individual days but worked off of midpoints (i.e., the "ides") and other divisions like "nones" (eight days before the ides) and "kalends" (the beginning of the month). It was complicated and unwieldy, which is why it lives on today only in Shakespeare's play.