Bissext, or bissextus (from Latin bis 'twice', and sextus 'sixth') is the 'leap day' which is added to the Julian calendar and the Gregorian calendar every fourth year to compensate for the six-hour difference in length between the common 365-day year and the actual length of the solar year. (The Gregorian calendar omits this leap day in years evenly divisible by 100, unless they are divisible by 400)
Originally, the day was inserted after 24 February, i.e. the 6th day before the calends (1st) of March, Consequently, besides the sextus, or sixth before the calends, the bis-sextus or "second sixth," was 25 February. In modern usage, with the exception of 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," or leap years.
Be it further enacted by the Authority aforesaid,
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.
- public domain: Chisholm, Hugh, ed. (1911). "Bissext". Encyclopædia Britannica. Vol. 4 (11th ed.). Cambridge University Press. p. 12. One or more of the preceding sentences incorporates text from a publication now in the
- Ruffhead, Owen (1763). "The Statute DE ANNO ET DIE BISSEXTILI made at Weſtminster 21 Anno Hen. III and Anno Dom. 1236". The Statutes at Large, from Magna Charta to the End of the Last Parliament 1761. Mark Basket. p. 20 – via Archive.org.
The day of the Leap Year, and the day before, shall be holden as one day
- Campion, Rev W M; Beamont, Rev W J (1870). The Prayer Book interleaved. London. 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.
- Pickering, Danby, ed. (1765). "Cap. XXIII: An act for regulating the commencement of the year; and for correcting the calendar now in use". The Statutes at Large: from the 23d to the 26th Year of King George II. Vol. 20. Cambridge: Charles Bathurst. (This is the original 1750/51 Act, in facsimile image. For clearer text, with long s ( ) converted to modern , see British Calendar Act of 1751, the original text of the 1750 Act in plain text (ASCII), from Wikisource.)
- Edward Coke (1628). "Cap. 1, Of Fee Simple.". First Part of the Institutes of the Lawes of England. p. 8 left .
... and by (b) the statute de anno bissextili, it is provided, quod computentur dies ille excrescens et dies proxime præcedens pro unico dii, (so as in computation that day excrescent is not accounted.)