What day of the week was (or will be) any date? Uses Zeller's congruence for the Gregorian calendar and supports dates from year 1 to 9999. Shows the ISO week number, day-of-year, days-until/since today, and a mini calendar for the selected month.
Zeller's congruence is a formula devised by Christian Zeller (1883) for computing the day of the week for any Julian or Gregorian calendar date. This tool uses the Gregorian variant:
h = (q + ⌊13(m+1)/5⌋ + K + ⌊K/4⌋ + ⌊J/4⌋ − 2J) mod 7, where h = 0 is Saturday.
ISO 8601 week numbering treats Monday as the first day and defines week 1 as the week containing the first Thursday of the year, so ISO week and year may differ from the calendar year near January/December boundaries.