Bissext, or bissextus is the leap day which is added to the Julian calendar every fourth year and to the Gregorian calendar almost every fourth year to compensate for the almost six hour difference in length between a common calendar year of 365 days and the average length of the solar year.[1]
In the ancient Julian calendar, 24February Latin: ante diem sextum Kalendas Martias [the 6th day before the [[calends]], or 1st of 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 Latin: ante diem bis sextum Kalendas Martias and the later one named Latin: ante diem sextum Kalendas Martias (the Latin word Latin: bis means 'second' or 'twice'). The calendar then proceeded as usual, with the day after both labelled Latin: ante diem quintum Kalendas Martias [fifth day before the Kalends] (25 February), as in common years.[2] The placement of the doubled day within this part of February was that formerly reserved for Mercedonius, the intercalary month that had been replaced by the single leap day.[3] In modern usage, with the exception of some ecclesiastical calendars, this intercalary day is added for convenience at the end of the month of February, as 29 February, and years in which February has 29 days are called "bissextile years" or leap years.[4]
Abandonment of the awkward practice of having two days with the same date appears to have evolved by custom and practice. 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. It was not until passage of the Calendar (New Style) Act 1750 that 29 February was formally recognised in British law.[5]
| Short Title: | Latin: Provisio de Anno Bisextili et Die |
| Type: | Act |
| Parliament: | Parliament of England |
| Long Title: | A Provision for the Day in Leap-Year. |
| Year: | 1256 |
| Citation: | 40 Hen. 3(Ruffhead: 21 Hen. 3) |
| Territorial Extent: | England |
| Repealing Legislation: | Civil Procedure Acts Repeal Act 1879 |
| Status: | repealed |
| Original Text: | https://babel.hathitrust.org/cgi/pt?id=pst.000017915496&seq=187 |
The term is first defined in English law, in the statute Latin: De Anno et Die Bissextili (Concerning [the] leap year and leap day, 40 Hen. 3, 1256), which defines the bissextile day as consisting of two actual days. (This was to clarify what should happen when "an essoin was given for a month" but the month was February in a leap year.[6]
Section II of the Calendar (New Style) Act 1750 (24 Geo. 2. c. 23) uses the word "bissextile" as a term for leap years.[7]