Bissext, or bissextus (from Latin bis 'twice' and sextus 'sixth') 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][2]
In the ancient Julian calendar, 24February – 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 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.[3] 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the doubled day within this part of February was that formerly reserved for Mercedonius, the intercalary month that had been replaced by the single leap day.[4] In modern usage, with the exception of some ecclesiastical calendars, this intercalary day is added for convenience at the end of the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.[1][5][a]
Abandonment of the awkward practice of having two days with the same date appears to have evolved by custom and practice.[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.[7]
The term is first defined in English law, in the statuteDe 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.[8]
Statute concerning [the] leap year and leap day
The day of the leap year, and the day before, shall be holden for one day
THE King unto his Justices of the Bench, greeting. Know ye, that where within our Realm of England, it was doubted of the Year and Day that were wont to be assigned unto such Persons being impleaded, when and from what Day of the Year going before unto another Day of the Year following, the Year and Day in a Leap Year ought to be taken and reckoned how long it was:
II. We therefore, willing that a Conformity be observed in this behalf every where within our Realm, and to avoid all Danger from such as be in Plea, have provided, and by the Counsel of our faithful Subjects have ordained, That, to take away from henceforth all Doubt and Ambiguity that might arise hereupon, the Day increasing in the Leap-Year shall be accounted for one Year, so that because of that Day none shall be prejudiced that is impleaded, but that it shall be taken and reckoned of the same month wherein it groweth; and that Day, and the Day next going before, shall be accounted for one Day. And therefore we do command you, that from henceforth you do cause this to be published afore you, and be observed. Witness myself at Westminster, &c .[2]
That the several Years of our Lord, 1800, 1900, 2100, 2200, 2300, or any other hundredth Years of our Lord, which shall happen in Time to come, except only every fourth hundredth [sic] Year of our Lord, whereof the Year of our Lord 2000 shall be the first, shall not be esteemed or taken to be Bissextile or Leap Years, but shall be taken to be common Years, consisting of 365 Days, and no more;
and that the Years of our Lord 2000, 2400, 2800, and every other fourth hundred Year of our Lord, from the said Year of our Lord 2000 inclusive, and also all other Years of our Lord, which by the present Supputation are esteemed to be Bissextile or Leap Years, shall for the future, and in all Times to come, be esteemed and taken to be Bissextile or Leap Years, consisting of 366 Days, in the same Sort and Manner as is now used with respect to every fourth Year of our Lord.
Notes
^Actual ecclesiastic practice varied. Campion (1870) reports the 23rd and the 25th being doubled instead at various times.[6] Pollard (1940) reports various dates in February being used in England; on at least one occasion, even a date in January was used.[5]: 188
^Pollard (1940) says "But in spite of Edward I's ordinance and Seymour's meticulous observance thereof as late as 1552, it is clear that the bi-sextile day had in common usage lost its etymological meaning".[5]: 188
^ abcdPollard, A F (1940). "New Year's Day and Leap Year in English History". The English Historical Review. 55 (218 (Apr., 1940)): 186. JSTOR553864.
^Campion, Rev W M; Beamont, Rev W J (1870). The Prayer Book interleaved. London: Rivingtons. p. 31 – via Archive.org. Before the Reformation St Matthias' day was kept in Leap-year, on Feb. 25th. In the Prayer-book of 1549 we read 'This is also to be noted, concerning the Leap-years, that the 25th day of February, which in Leap-years is counted for two days, shall in those two days alter neither Psalm nor lesson; but the same Psalms and Lessons which be said the first day, shall also serve for the second day'." Wheatly thinks that this alteration was made in order that the Holy day might always be kept on the 24th. In the Calendar put forth in 1561 the old practice was resumed, and the following rule which was inserted in the Prayer-book of 1604, was promulgated: "When the year of our Lord may be divided into four even parts, which is every fourth year, then the Sunday letter leapeth, and that year the Psalms and Lessons which serve for the 23rd day of February, shall be read again the day following, except it be Sunday, which hath Proper Lessons of the Old Testament, appointed in the Table serving to that purpose." In 1662, the intercalary day was made the 29th of February so that St Matthias now must always be kept on the 24th.
