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Bissext, or bissextus () is the 'leap day' which is added to the
Julian calendar The Julian calendar, proposed by Roman consul Julius Caesar in 46 BC, was a reform of the Roman calendar. It took effect on , by edict. It was designed with the aid of Greek mathematicians and astronomers such as Sosigenes of Alexandri ...
and the
Gregorian calendar The Gregorian calendar is the calendar used in most parts of the world. It was introduced in October 1582 by Pope Gregory XIII as a modification of, and replacement for, the Julian calendar. The principal change was to space leap years d ...
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 A tropical year or solar year (or tropical period) is the time that the Sun takes to return to the same position in the sky of a celestial body of the Solar System such as the Earth, completing a full cycle of seasons; for example, the time f ...
. (The Gregorian calendar omits this leap day in years evenly divisible by 100, unless they are divisible by 400) In the
Julian calendar The Julian calendar, proposed by Roman consul Julius Caesar in 46 BC, was a reform of the Roman calendar. It took effect on , by edict. It was designed with the aid of Greek mathematicians and astronomers such as Sosigenes of Alexandri ...
, 24 February i.e. the 6th day before the
calends The calends or kalends ( la, kalendae) is the first day of every month in the Roman calendar. The English word "calendar" is derived from this word. Use The Romans called the first day of every month the ''calends'', signifying the start of a ...
(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. Consequently the , or sixth before the calends, the or "second sixth," was ''also'' 24 February. In modern usage, with the exception of some ecclesiastical calendars, this
intercalary day Intercalation or embolism in timekeeping is the insertion of a leap day, week, or month into some calendar years to make the calendar follow the seasons or moon phases. Lunisolar calendars may require intercalations of both days and months. ...
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 year A leap year (also known as an intercalary year or bissextile year) is a calendar year that contains an additional day (or, in the case of a lunisolar calendar, a month) added to keep the calendar year synchronized with the astronomical year or ...
s. Replacement (by 29 February) 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 The House of Commons of England was the lower house of the Parliament of England (which incorporated Wales) from its development in the 14th century to the union of England and Scotland in 1707, when it was replaced by the House of Commons ...
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. Section II of the Calendar (New Style) Act 1750 uses the word "bissextile" as a term for leap years. (This is the original 1750/51 Act, in facsimile image. For clearer text, with
long s The long s , also known as the medial s or initial s, is an archaic form of the lowercase letter . It replaced the single ''s'', or one or both of the letters ''s'' in a 'double ''s sequence (e.g., "ſinfulneſs" for "sinfulness" and "po� ...
() converted to modern , see
British Calendar Act of 1751 The Calendar (New Style) Act 1750 (24 Geo. II c.23), also known as Chesterfield's Act or (in American usage) the British Calendar Act of 1751, is an Act of the Parliament of Great Britain. Its purpose was for Great Britain and t ...
, the original text of the 1750 Act in plain text (ASCII), from
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.)


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Further reading

*{{Cite book , last=
Edward Coke Edward is an English given name. It is derived from the Anglo-Saxon name ''Ēadweard'', composed of the elements '' ēad'' "wealth, fortune; prosperous" and '' weard'' "guardian, protector”. History The name Edward was very popular in Anglo-Sax ...
, title=First Part of the Institutes of the Lawes of England , date=1628 , page=8 left 0, chapter=Cap. 1, ''Of Fee Simple.'' , quote=... and by (b) the statute {{lang, la, de anno bissextili, it is provided, {{lang , chapter-url=https://archive.org/details/firstpartofinsti011628coke/page/2/mode/1up , la , quod computentur dies ille excrescens et dies proxime præcedens pro unico dii, ({{transl , la , so as in computation that day excrescent is not accounted.) Julian calendar