Day of the Week Calculator
The weekday of any date from year 1 to 9999, with the working shown, plus whether its year is a leap year, the day of the year and the ISO week.
Result…
How it's calculated
Zeller: h = (q + ⌊13(m + 1) ÷ 5⌋ + K + ⌊K ÷ 4⌋ + ⌊J ÷ 4⌋ + 5J) mod 7, 0 = Saturday January and February are months 13 and 14 of the year before · K = year mod 100, J = ⌊year ÷ 100⌋ Leap year: divisible by 4, except centuries not divisible by 400
Finds the weekday with Zeller's congruence on the proleptic Gregorian calendar (the modern calendar carried back to year 1), and checks it the same way a computer does, by counting days. A year is a leap year if it divides by 4, except century years, which must divide by 400: 2000 was a leap year, 1900 was not. The ISO week follows ISO 8601: weeks start on Monday and week 1 is the week with the year's first Thursday, so the first days of January can belong to the last week of the year before. To add or count days between two dates, use the date calculator.
Your numbers, step by step
About the Day of the Week calculator
This day of the week calculator tells you which weekday any date falls on, from 1 January of year 1 to 31 December 9999. Enter a birthday, a historical event or a future deadline and it names the day, then shows the working with Zeller's congruence, a short formula that turns the day, month and year into a number from 0 to 6. It checks that answer a second way by counting days from 1 January 1970. You also get whether the year is a leap year and why, the day number of the year, the days left in the year and the ISO 8601 week, the numbering many diaries and planners use. Add a second date to see how many days apart the two are. Every date uses the modern Gregorian calendar, carried back before it was adopted.
How to use it
- Enter the “Date” you want to look up.
- Leave “Count the days from” at today, or pick another date to compare with.
- Read the weekday, then the leap year, day of the year, days left and ISO week below it.
- Open the steps to follow Zeller's congruence with your own day, month and year.
Worked example
1 January 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, and 0 means Saturday. 2000 divides by 400, so it is a leap year with 366 days; 1 January is day 1, with 365 days left. Its ISO week is 1999-W52-6, because the Thursday of that week falls in 1999. Counted from 27 October 2026, it was 9,796 days earlier.
Updated
Questions people ask
How do I work out the day of the week for any date?
Use Zeller's congruence: h = (q + ⌊13(m + 1) ÷ 5⌋ + K + ⌊K ÷ 4⌋ + ⌊J ÷ 4⌋ + 5J) mod 7, where q is the day, m the month, K the last two digits of the year and J the century. January and February count as months 13 and 14 of the year before. h = 0 is Saturday, 1 Sunday and so on to 6, Friday.
How do I know if a year is a leap year?
A year is a leap year if it divides by 4, except century years, which must also divide by 400. So 2024 and 2000 were leap years, while 1900 was not and 2100 will not be.
Why does early January sometimes show last year's week number?
ISO 8601 weeks start on Monday, and week 1 is the week that holds the year's first Thursday. If 1 January falls on a Friday, Saturday or Sunday, those days belong to the last week of the year before; 1 January 2027 is in week 53 of 2026.
What day of the year is it?
Add the days in each month before the date, then the day of the month. 1 March is day 60 in an ordinary year and day 61 in a leap year.
Is the weekday right for very old dates?
It is right on the Gregorian calendar. Many records before 1582, and in some countries until the 1900s, use the older Julian calendar, which by then ran 10 to 13 days behind, so an old date can name a different weekday. Check which calendar your source uses. To add or count days between dates, use the date calculator.