Day of the Week Finder

How to use the day of the week finder

1. Pick a Date. Any date from January 1 of year 1 to December 31, 9999 works. Today brings back today’s date.
2. Read the Day of the week, the Full date, the Day of the year and the ISO week.
3. Under Step by step with Zeller’s congruence, follow the calculation by hand, with every intermediate value.
4. In What day was I born?, enter your Date of birth to see the weekday you were born on, how many days ago that was, which day your next birthday falls on, and how many of your birthdays so far fell on the same weekday.
5. The same date in other years shows which weekday that day and month falls on in each year between From year and To year. It is handy for birthdays, anniversaries and recurring events. The chosen year is highlighted.

Zeller’s congruence explained

Christian Zeller published this formula in 1882 to compute the weekday of any Gregorian date by hand:

h = (q + ⌊13(m + 1) / 5⌋ + K + ⌊K / 4⌋ + ⌊J / 4⌋ + 5J) mod 7

• q is the day of the month;
• m is the month, counted from March (3) to December (12), with January and February counted as months 13 and 14 of the previous year — this moves the leap day to the end of the year;
• K is the year within the century (year mod 100) and J the century (year divided by 100, rounded down);
• ⌊x⌋ means “round down”, and mod 7 is the remainder after dividing by 7.

The result h gives the weekday: 0 = Saturday, 1 = Sunday, 2 = Monday and so on up to 6 = Friday.

Example for January 1, 2000: January counts as month 13 of 1999, so q = 1, m = 13, K = 99 and J = 19. Then h = (1 + 36 + 99 + 24 + 4 + 95) mod 7 = 259 mod 7 = 0, which is a Saturday. This tool shows the same steps for the date you choose, and checks the result against an independent day count.

Dates before 1582

The Gregorian calendar was introduced on Friday, October 15, 1582; the day before was Thursday, October 4 in the older Julian calendar. For earlier dates, the tool uses the proleptic Gregorian calendar, the current rules extended backwards, which is the ISO 8601 convention. It warns you and gives the Julian date of the same day, since documents of the time used that calendar. Some countries switched much later: Great Britain and its colonies in 1752, Russia in 1918. A date copied from an old British or Russian source may therefore be a Julian date.

The weekday of a given day never changes between the two calendars, only its date does. For example, William Shakespeare died on April 23, 1616 (Julian), which is May 3, 1616 in the Gregorian calendar; both were a Tuesday.