“Romans always reconciled differences between calendar and solar year lengths during the “Month of Purification.” Whenever and however Roman calendars were modified to correspond to year length, it was always done after the 23rd day of February, traditionally the last day of the year. Even in our time, leap year is observed with a 29-day February. To purists, “leap day” is February 24, not the 29th.”
More about the early Roman calendar.
Three times in history a February 30th has also been in the calendar of selected countries! Sometimes when I think of the stuff that I need to get done this month, I wish this year it happend for a fourth time.