Whether you were born on a Monday or a Saturday is more than trivia—it is a classic problem in calendar mathematics that occupied mathematicians for centuries until Christian Zeller formulated his congruence equation in 1882.
Zeller's Congruence Algorithm
For Gregorian calendar dates, Zeller's formula calculates the weekday index h (where 0 = Saturday, 1 = Sunday, ..., 6 = Friday):
h = (q + floor(13*(m+1)/5) + K + floor(K/4) + floor(J/4) - 2*J) mod 7
Where q is the day of the month, m is the month (with March=3, ..., December=12, January=13 of prev year, February=14 of prev year), K is the year of the century (year mod 100), and J is the zero-based century (floor(year/100)).
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