[1][2] In the ancient Julian calendar, 24 February – ante diem sextum Kalendas Martias, the 6th day before the calends, or 1 March, counting backwards inclusively in the Roman style (1/3, 28/2, 27/2, 26/2, 25/2, 24/2) – was doubled in a leap year.

Both days had the same date, the earlier one called ante diem bis sextum Kalendas Martias and the later one named ante diem sextum Kalendas Martias (the Latin word bis means 'second' or 'twice').

The calendar then proceeded as usual, with the day after both labelled ante diem quintum Kalendas Martias [fifth day before the Kalends] (25 February), as in common years.

[b] In the course of the fifteenth century, "29 February" appears increasingly often in legal documents – although the records of the proceedings of the House of Commons of England continued to use the old system until the middle of the sixteenth century.

[5]: 188  It was not until passage of the Calendar (New Style) Act 1750 that 29 February was formally recognised in British law.