This is an interesting algorithm that uses module theory and congruences to compute on what day of the week will a certain date (DD,MM,YYYY) coincide.
h = (q + ⌊(13(m + 1)) / 5⌋ + K + ⌊K / 4⌋ + ⌊J / 4⌋ - 2J) mod 7
Where:
- h is the day of the week (0 = Saturday, 1 = Sunday, ..., 6 = Friday)
- q is the day of the month
- m is the month (January and February are counted as months 13 and 14 of the previous year) This I will explain later on
- K is the year of the century (
year % 100) - J is the zero-based century (
(year / 100))
Why are January and February counted as months 13 and 14 of the previous year?
- For an easier year transition as the transition between years (from December to January) would require additional logic to handle the change in the year part of the formula.