A leap year, in the Gregorian and Julian calendars, is a year containing 366 days instead of the ordinary 365. Normally, this occurs every four years, where February has an extra day, bringing the number of days in that month to 29. Leap years occur when a year's number is a multiple of 4 (1992, 2144, 1852, etc.). However, when a year's number is a multiple of 100, it is only a leap year if the number is also a multiple of 400. Thus 1900 was not a leap year while 2000 was and 2100 will not be.