Day of the Week Calculator
Find the weekday of any date, with its day number in the year, its ISO week and whether the year is a leap year.
Day of the week
Type a date as YYYY-MM-DD.
Day of the week
Saturday
Full date
Saturday, 1 January 2000
Day 1 of 366
Day of the year
1999-W52
ISO week
2000 is a leap year
Leap year?
365 days left in 2000
Days left in the year
Private: Everything runs in this page. Nothing you type is sent anywhere or saved.
On this page
Weekday examples
| Date | Weekday | Day of the year |
|---|---|---|
| 1970-01-01 | Thursday | Day 1 of 365 |
| 1969-07-20 | Sunday | Day 201 of 365 |
| 2000-01-01 | Saturday | Day 1 of 366 |
| 2024-02-29 | Thursday | Day 60 of 366 |
| 1900-01-01 | Monday | Day 1 of 365 |
Formula
weekday = (day number + 4) mod 7, counting from Sunday = 0, where 1970-01-01 (a Thursday) is day 0
How the day of the week works
- Step oneType the date as YYYY-MM-DD.
- Step twoRead the weekday in large letters.
- Step threeThe tiles show the day of the year, the ISO week and whether the year is a leap year.
What this tool does not do
- It uses the Gregorian calendar for every date, so dates before it was adopted in a country can differ from old records.
- It handles the years 1000 to 9999.
- It does not know holidays.
Frequently asked questions
Which weekday was January 1, 1970?
A Thursday. It is day 0 of Unix time, which is why the formula starts from it.
Does it work for dates before the Gregorian calendar?
It uses the Gregorian calendar for all dates (years 1000 to 9999). Places that still used the Julian calendar then had a different weekday date, so old dates can differ from historical records.
What does the ISO week show?
The week number as ISO 8601 counts it: weeks start on Monday and week 1 is the week with the first Thursday of the year.