Year "year", originally 秂, an ideographic compound of a person carrying a bundle of wheat denoting "harvest". Slavic besides "godŭ" "time period; year" uses "lěto" "summer; year". In the International System of Quantities (ISO 80000-3), the year (symbol, "a") is defined as either 365 days or 366 days. No astronomical year has an integer number of days or lunar months, so any calendar that follows an astronomical year must have a system of intercalation such as leap years. In the Julian calendar, the average (mean) length of a year is 365.25 days. In a non-leap year, there are 365 days, in
February 29 February 29 February 29, also known as leap day or leap year day, is a date added to most years that are divisible by 4, such as 2012, 2016, 2020, 2024, and 2028. A leap day is added in various solar calendars (calendars based on the Earth's revolution around the Sun), including the Gregorian calendar standard in most of the world. Lunisolar calendars (whose months are based on the phases of the Moon) instead add a leap or intercalary month. In the Gregorian calendar, years that are divisible by 100, but not by 400, do not contain a leap day.