A leap year is a year containing one additional day added to keep the calendar year synchronized with the astronomical or seasonal year. Because seasons and astronomical events do not repeat in a whole number of days, calendars that have the same number of days in each year drift over time with respect to the event that the year is supposed to track. By inserting an additional day or month into the year, the drift can be corrected. A year that is not a leap year is called a common year.
For example, in the Gregorian calendar, each leap year has 366 days instead of the usual 365, by extending February to 29 days rather than the common 28.
The name "leap year" comes from the fact that while a fixed date in the Gregorian calendar normally advances one day of the week from one year to the next, the day of the week in a leap year will advance two days (from March onwards) due to the extra day added at the end of February (thus "leaping over" one of the days in the week). For example, Christmas fell on Tuesday in 2001, Wednesday in 2002, and Thursday in 2003 but then "leapt" over Friday to fall on a Saturday in 2004.