Leap year

A calendar year with an extra day or month inserted to keep the calendar aligned with the astronomical year. In the Gregorian calendar, it has 366 days.

Gregorian calendar: The solar calendar used in most of the world, with 365-day common years and leap years governed by century rules. Its 400-year rule determines which years gain February 29.
Julius Caesar: Roman dictator who introduced the Julian calendar in 46 BCE. His calendar established the every-fourth-year leap rule in Roman civic life.
Hebrew calendar: A lunisolar calendar that inserts a leap month in seven years of each 19-year cycle. Its leap years add Adar I to keep lunar months near their seasons.
February 29 birthday: A birthday falling on February 29, a date present only in Gregorian leap years. Its rarity raises practical questions about when the birthday is observed in common years.
February 29: The additional date in the Gregorian calendar during a leap year. This is the day inserted to extend a Gregorian year.
Sosigenes of Alexandria: An Alexandrian astronomer traditionally credited with advising Julius Caesar on calendar reform. He is associated with the astronomical advice behind the Julian system.
Islamic calendar: A lunar calendar of 12 months whose ordinary year is about 354 days. It does not add leap months to keep its months aligned with the solar seasons.
Day count convention: A method for counting days between dates in financial calculations. Leap days affect year fractions under some interest and securities conventions.
Tropical year: The average interval between successive vernal equinoxes, about 365.2422 days. Its fractional length creates the need for periodic calendar correction.
Council of Nicaea: A gathering of Christian bishops convened by Constantine in 325 CE. Its Easter date rules made seasonal accuracy in the Julian calendar consequential.

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