What Day of the Week Was I Born?

Any date, any year, the weekday it fell on, plus the Monday's Child line, your next birthday's weekday, and how the arithmetic works.

Monday's Child

The rhyme that goes with this question was first printed in A. E. Bray's Traditions of Devonshire in 1838, though it is older in oral form. The calculator shows the line for your day. If you were born on a Wednesday, sorry; the rhyme has been unkind to Wednesday for two centuries and nobody has fixed it.

How the weekday is found

Every date has a fixed weekday because the calendar is a long, regular count of days. The calculator counts days from a known anchor and takes the remainder when dividing by seven. You can do the same by hand with John Conway's Doomsday rule: in any given year, the dates 4/4, 6/6, 8/8, 10/10, 12/12, the last day of February, 5/9, 9/5, 7/11 and 11/7 all land on the same weekday (the year's "doomsday"). Learn that one weekday for the year and you can get any other date by counting from the nearest of those anchors. The "Show the work" box prints the year's doomsday and the count so you can follow it.

Calendar caveats

The results assume the Gregorian calendar, which is what the entire world uses now, but it was adopted at different times. Catholic Europe switched in 1582; Britain and its colonies (including what became the United States) in September 1752, when eleven days were skipped; Russia in 1918; Greece in 1923. For a birth before the switch in a given country, the recorded date was Julian and the weekday on the document may differ from what this calculator says. George Washington was born 11 February 1731 Julian, which is 22 February 1732 Gregorian; both were Fridays, as it happens.

Why people ask

Baby books, the rhyme, horoscopes that assign planets to weekdays (Sunday to the Sun, Monday to the Moon, and so on through Saturn's day), and plain curiosity. Some cultures mark the birth weekday formally: Akan names from Ghana such as Kofi (Friday) and Kwame (Saturday) record it directly, and Thai Buddhist tradition assigns a Buddha posture and colour to each day of birth.

Doomsday rule: J. H. Conway, "Tomorrow is the Day After Doomsday," Eureka 36 (1973).