Zeller's Congruence is an algorithm devised by Christian Zeller to calculate the day of the week for any Julian or Gregorian calendar date.
Zeller's Algorithm can be used to determine the day of the week for any date in the past, present or future.
Actually the algorithm should work only for those dates starting with the use of the Gregorian Calendar in the year 1582. The Gregorian Calendar will be off one full day in the year 4902, so this algorithm works for any dates between