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@brooke This is because in the pre-Julian Roman calendar, that's where February was interrupted for the intercalary month Mercedonius. (24 days of February, 30? of Mercedonius, 4 more days of February.)

And although the distinction is largely elided these days, Leap Day being February 25th is still important in some places - there's at least one country (Sweden?) where that's in the laws, and some liturgical calendars rely on it.