Leap years present a problem for the calendar of saints days. In the ancient Roman calendar introduced by Julius Caesar, which was the basis of the medieval calendar, the extra day in a leap year was inserted as the "second sixth day" before the Kalends of March -- that is, the day after the 23rd day of February. This continued as a general practice in the Middle Ages. The day inserted after the 23rd was empty of saints, so the Feast of Matthias was moved from what, in a leap year, would be the 25th of February to fill the blank created by Leap Day (the 24th) [see GTZ, p. 16]. Some sources indicate that the insertion of the leap day resulted in a doubling of the day-letter (i.e. abcdeffg instead of abcdefg). If anyone knows more about this, please send me a note.
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