Leap days are the extra days (February 29) that are added to the Gregorian calendar in leap years.

Since each 400 years of the Gregorian calendar has

days, of which 97 are leap days, the probability P of being a leap day baby is

The following table summarizes the years in which the leap day falls on given weekdays for the years 1900-2100.

weekday leap years having that weekday at the leap day Sunday 1920, 1948, 1976, 2004, 2032, 2060, 2088 Monday 1904, 1932, 1960, 1988, 2016, 2044, 2072 Tuesday 1916, 1944, 1972, 2000, 2028, 2056, 2084 Wednesday 1928, 1956, 1984, 2012, 2040, 2068, 2096 Thursday 1912, 1940, 1968, 1996, 2024, 2052, 2080 Friday 1924, 1952, 1980, 2008, 2036, 2064, 2092 Saturday 1908, 1936, 1964, 1992, 2020, 2048, 2076

If birthdays are distributed randomly throughout the calendar, then there would be approximately four million leap day babies in the world (assuming a world population of six billion). However, as the figure above and chart below show, some refinement of this estimate is needed since there is a decided peak in September for births in the United States between 1978-1987 (Peterson 1998).

month average daily birth frequency January 0.0026123 February 0.0026785 March 0.0026838 April 0.0026426 May 0.0026702 June 0.0027424 July 0.0028655 August 0.0028954 September 0.0029407 October 0.0027705 November 0.0026842 December 0.0026864

The premise of Gilbert and Sullivan's musical The Pirates of Penzance is based on the birth of the main character on a leap day. In more modern times, there is website devoted to leap day babies.

Gregorian Calendar, Leap Year, Millennium









Honor Society of Leap Day Babies. "Leap Day Babies." http://www.leapzine.com/hr/ . Peterson, I. "MathTrek: Birthday Surprises." Nov. 21, 1998. http://www.sciencenews.org/sn_arc98/11_21_98/mathland.htm . Starr, A. "Leap Day/Leap Year." http://www.emailman.com/leapday/ . Strohsacker, J. "@ February 29 Leap Day." http://www.mystro.com/leap.htm .







© 1996-2007 Eric W. Weisstein