February 30 occurs in some calendars, but not in the Gregorian calendar, where February contains only 28 or in a leap year, 29 days.
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The Swedish Empire (which included Finland at the time) planned to change from the Julian calendar to the Gregorian calendar beginning in 1700 by omitting leap days for the next 40 years. Although the leap day was omitted in February 1700, the Great Northern War began later that year, diverting the attention of the Swedes from their calendar so they did not omit leap days on the next two occasions, causing 1704 and 1708 to remain leap years.
To avoid confusion and further mistakes, the Julian calendar was restored when, in 1712, one extra leap day was added, thus giving that year a 30th of February. That date corresponded to February 29 in the Julian calendar and to March 11 in the Gregorian calendar. The Swedish changeover to the Gregorian calendar was finally accomplished in 1753 by omitting the last 11 days of February.
Although many sources erroneously state that 30-day months were used in the Soviet Union for part or all of the period 1929–1940, in fact the Soviet calendar with 5-day and 6-day weeks was only used for planning the workdays and days of rest. The traditional calendar remained for everyday use - surviving physical calendars from that period only show the irregular months of the Gregorian calendar, including a 28- or 29-day February, so there was never a February 30.[1]
The 13th century scholar Sacrobosco claimed that in the Julian calendar February had 30 days in leap years between 45 BC and 8 BC, when Augustus shortened February to give the month of August named after him the same length as the month of July named after his adoptive father Julius Caesar. In fact, if ordinary February 24 was ante diem sextum Kalendas (the sixth day before the beginning of March), February 28 was ante diem secundum Kalendas (the second day before the beginning of March), while February 29 was ante diem (primum) Kalendas (the day before the beginning of March). During leap years, the extraordinary day was added after Febrauary 24, being called ante diem bis sextum Kalendas (the second sixth day before the beginning of March) from whom derived the word bis-sextum which was used to indicate leap years in Neo-Latin languages (French: année bissextile, Spanish: año bisiesto, Italian: anno bisestile).
Artificial calendars may also have 30 days in February. For example, in a climate model the statistics may be simplified by having 12 months of 30 days. The Hadley Centre General Circulation Model is an example.
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