March 0

March 0 is a fictitious day that would be located between February 28 (29 in leap years) and March 1. Like the also fictitious January 0, it is represented in Microsoft Excel and can be used to refer to the last day of February, because the duration of this month varies according to whether or not it is a leap year. Also, sometimes it is used by astronomers as a synonym of the last day of February.

March 0 is used in the Doomsday Algorithm to calculate on what day of the week a day falls for a given year.

wiki