Day of the Week Calculator
Find what day of the week any date falls on, past or future, plus its day of the year, ISO week number, quarter, leap-year status and how many days it is from today. Example: January 1, 2000 was a Saturday.
Show steps
How to use the Day of the Week Calculator
- Pick a date with the date field.
- Press Calculate to see the weekday and calendar details.
Formula and method
Zeller’s congruence or the Doomsday rule find the weekday by counting days modulo 7. For the Gregorian calendar: h = (q + floor(13(m+1)/5) + K + floor(K/4) + floor(J/4) + 5J) mod 7, where h = 0 is Saturday.
Famous dates and their weekdays
| Date | Weekday |
|---|---|
| January 1, 2000 | Saturday |
| July 4, 1776 | Thursday |
| July 20, 1969 (Moon landing) | Sunday |
| December 25, 2025 | Thursday |
| January 1, 2030 | Tuesday |
Leap year rules
| Rule | Example |
|---|---|
| Divisible by 4 is a leap year | 2024 |
| Except centuries are not | 1900 is not |
| Unless divisible by 400 | 2000 is |
Doomsday trick
Every year has fixed dates that share a weekday, the doomsday: 4/4, 6/6, 8/8, 10/10, 12/12, 5/9, 9/5, 7/11 and 11/7. Know the doomsday for the year and you can count to any other date.
Uses
Planning events, checking birth days, verifying deadlines and finding which day an anniversary falls on.
Frequently asked questions
What day of the week was I born?
Enter your birth date and the calculator shows it.
How is the weekday calculated?
By counting days from a reference date, modulo 7.
What is the ISO week number?
A week-numbering system where weeks start on Monday and week 1 contains the first Thursday.
Is this correct for old dates?
It uses the Gregorian calendar extended backwards, which is the modern standard.
What is a leap year?
A year with 366 days, added to keep the calendar aligned with Earth’s orbit.
How many days until a date?
The calculator shows the distance from today in days and weeks.
Sources
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