The warning to "beware the ides of March" originated from the assassination of Julius Caesar on March 15 in 44 BCE, but what the heck is (are?) ides anyway?

Ides is an old Roman calendar term used to denote a full moon, so every month had an ides, not just March. (In rare cases, what we call Blue Moons, some months would have had two ides.)

The Romans at first based their calendar at first on the phases of the moon: Kalends started on the new moon, Nones began on the first quarter, and Ides started on the day of the full moon. A lunar year has some drawbacks, though, not least of which is that it doesn't match up evenly with a solar year. So it didn't take long for seasonal events like festivals to be seriously out of whack with the official calendar.

Julius Caesar implemented calendar reform, setting the length of the year at 365 days (plus a leap day every 4 years) and fixing the lengths of the months and setting the ides of March, May, July and October on the 15th and the other months, on the 13th.

In the Christian era, the Julian calendar eventually eas replaced by the Gregorian calendar, in large part to make calculating the date of Easter somewhat simpler. Like Passover, the date of Easter is related to lunar cycles, which is why it jumps around on the calendar so much. Further adding to the confusion is the fact that Eastern Churches and Western Churches calculate Easter differently, so their celebrations only rarely coincide.