Day of the Week Calculator
Pick any date from year 1 to 9999 to see which weekday it falls on, how many of those weekdays have passed since, and the full math behind it using Zeller's congruence and the Doomsday rule.
Method 1: Zeller's congruence
Method 2: Conway's Doomsday rule
How to use the Day of the Week Calculator
- Choose the month, then type the day and the year.
- Pick a calendar: Gregorian for modern dates, Historical or Julian for dates before 1582.
- Read the weekday along with the days elapsed and how many of that weekday have passed.
- Check the calendar preview and the list of upcoming years with the same weekday.
- Follow the two methods to see how the answer is calculated by hand.
Zeller's congruence, worked example
For the Gregorian calendar the formula is h = (q + ⌊13(m + 1)/5⌋ + K + ⌊K/4⌋ + ⌊J/4⌋ + 5J) mod 7, where q is the day of the month, m the month (March = 3 … December = 12, January = 13, February = 14 of the previous year), K the year of the century and J the century. The result h is 0 for Saturday, 1 for Sunday, 2 for Monday and so on.
Example: 1 January 2000. January counts as month 13 of 1999, so q = 1, m = 13, K = 99, J = 19. Then h = (1 + ⌊182/5⌋ + 99 + ⌊99/4⌋ + ⌊19/4⌋ + 95) mod 7 = (1 + 36 + 99 + 24 + 4 + 95) mod 7 = 259 mod 7 = 0, which is Saturday.
For the Julian calendar replace ⌊J/4⌋ + 5J with 5 + 6J.
Doomsday rule cheat sheet
| Century | Anchor day (Gregorian) | Month | Doomsday date |
|---|---|---|---|
| 1700s | Sunday | January | 3rd (4th in leap years) |
| 1800s | Friday | February | 28th (29th in leap years) |
| 1900s | Wednesday | March | 14th (also "March 0" = last day of Feb) |
| 2000s | Tuesday | April, June, August, October, December | 4/4, 6/6, 8/8, 10/10, 12/12 |
| 2100s | Sunday | May, September | 9th of May, 5th of September ("9-to-5 at 7-11") |
| 2200s | Friday | July, November | 11th of July, 7th of November |
To find a year's doomsday, take the last two digits yy and add ⌊yy/12⌋ + (yy mod 12) + ⌊(yy mod 12)/4⌋ to the century anchor, then reduce mod 7. Example for 2026: 2 + 2 + 0 = 4 days after Tuesday, so the doomsday is Saturday. That makes 10 October 2026 a Saturday, and 19 October 2026 (9 days later) a Monday.
Gregorian vs Julian calendar
- Julian calendar (from 45 BC): every fourth year is a leap year, so the average year is 365.25 days, about 11 minutes too long.
- Gregorian calendar (from 15 October 1582): century years are leap years only if divisible by 400 (1600 and 2000 were, 1700, 1800 and 1900 were not). The average year is 365.2425 days.
- The switch: in Catholic countries Thursday 4 October 1582 was followed by Friday 15 October 1582. The weekday cycle continued unbroken. Britain and its American colonies switched in September 1752, skipping 11 days.
Frequently asked questions
What day of the week was I born?
Enter your date of birth and the calculator shows the weekday, for example 20 July 1969 was a Sunday. It also counts how many of that weekday have passed since you were born and shows the line from the "Monday's child" nursery rhyme.
How does Zeller's congruence work?
It turns a date into a number from 0 to 6 using the day, month, year of the century and century, with January and February treated as months 13 and 14 of the previous year. In Zeller's numbering, 0 is Saturday, 1 is Sunday, and so on up to 6 for Friday.
What is the Doomsday rule?
It is a mental method by John Conway. Certain easy dates, such as 4/4, 6/6, 8/8, 10/10, 12/12 and the last day of February, always fall on the same weekday in a given year, called the doomsday. Find the year's doomsday, then count from the nearest one.
Why do dates before 1582 have two possible weekdays?
The Gregorian calendar replaced the Julian calendar in October 1582, when 10 days were skipped. Historical sources before that date use the Julian calendar, so choose Historical or Julian to match them. Gregorian mode extends today's calendar backwards (proleptic).
How often does a date fall on the same weekday?
A date usually repeats its weekday after 6, 11 or 5 years in a repeating pattern, because a common year shifts dates by one weekday and a leap year by two. The full Gregorian cycle repeats exactly every 400 years.
Is the result based on my time zone?
The weekday of a calendar date does not depend on time zones. Only the "days since" and "today" figures use your device's current date.