Day of the week calculator

The weekday comes from Zeller's congruence, and for Gregorian dates the Doomsday rule works the same answer a second way; both workings are shown so you can check them by hand. Historical days are answered in the calendar you pick: a Julian date and the same numbers read as Gregorian are different physical days, and this tool never deletes the days Britain removed in September 1752 or Catholic Europe removed in October 1582, it just tells you which calendar owns them. Weekday numbering is cyclic arithmetic, so the answers for any year between 1 and 3000 are exact; the tool does not handle BC years, time zones, or calendar reform beyond the two cutovers.