A leap year (or intercalary year) is a year containing one or more extra days in order to keep the calendar
year synchronised with the astronomical or seasonal year.

For example, in the Gregorian calendar, February has 29 days in a leap year instead of the usual 28 -
and consequently, the year lasts 366 days instead of the common 365.

Because seasons and astronomical events do not repeat at an exact number of full days,
a calendar that had the same number of days in each year would,
over time, drift with respect to the event it was supposed to track.

By occasionally inserting (or intercalating) an additional day or month into the year, the drift can be corrected.