In the Gregorian calendar, an end-of-century leap year (often referred to as a century leap year) is a year that is exactly divisible by 400 and, as with every other leap year, qualifies for the intercalation of February 29. End-of-century years that are not divisible by exactly 400 are common years, and not leap years. The years 2000 and 2400 for example, are end-of-century leap years in a century with 36,525 days, while 1700, 1800, 1900, 2100, 2200, 2300 and 3000 are common years in a century with 36,524 days. Leap years divisible by 400 always start on a Saturday, and thus the resulting upcoming February 29 (the leap day) is always on a Tuesday. The start days of the end-of-century years that are common years varies; the century year begins on a Friday if the decimal value is .25 when dividing it by 400, Wednesday if the decimal is .5, and Monday for decimal values of .75.
The end-of-century year "divisible by 400" rule of the Gregorian calendar was considered an improvement over the previously utilized Julian calendar which had provided for a leap year at four year intervals. Over time, the Julian practice resulted in too many leap days being added to the calendar, thus causing it to gradually drift with respect to the astronomical seasons of the years (and natural events, such as the spring equinox, to occur earlier and earlier in the calendar).
- Spofford, Thomas (1835). A new system of practical astronomy: made plain and easy to those who have not studied mathematics : containing the elementary principles of the science, all the rules and tables necessary for making all the calculations for an almanac …. Boston: Lemuel Gulliver. p. 28.