A perpetual calendar employs a table for finding which of fourteen yearly calendars to use. A table for the Gregorian calendar expresses its 400-year grand cycle: 303 common years and 97 leap years total to 146,097 days, or exactly 20,871 weeks. This cycle breaks down into one 100-year period with 25 leap years, making 36,525 days, or "one" day less than 5,218 full weeks; and three 100-year periods with 24 leap years each, making 36,524 days, or "two" days less than 5,218 full weeks.