About the Day of the Week Calculator
This calculator finds which day of the week any date falls on, past or future. It is useful for planning, for checking historical dates, and for settling the perennial question of what day you were born.
The formula
Zeller's congruence / built-in calendar arithmeticThe calculation uses the Gregorian calendar. Zeller's congruence is the classical formula for computing the weekday from a date arithmetically.
How to use this calculator
- 1Enter your Year. The field starts at
2026, which you can overwrite. - 2Enter your Month (1–12). The field starts at
8, which you can overwrite. - 3Enter your Day. The field starts at
11, which you can overwrite. - 4Read the result straight away — it recalculates as you type, so there is no button to press. Use Share to copy a link that reopens the page with your exact numbers filled in.
Worked example
| Input | Value |
|---|---|
| Year | 2026 |
| Month (1–12) | 8 |
| Day | 11 |
Result
Date: August 11, 2026
Day of Week: Tuesday
Day of Year: 223
Week of Year: 32
Leap Year: No
Those are the values the page loads with, so you can reproduce this result yourself and then change one field at a time to see what drives the outcome.
Understanding your result
The Gregorian calendar repeats on a 400-year cycle containing exactly 146,097 days, which is divisible by 7. That means the pattern of weekdays repeats exactly every 400 years — 1 January 2000 and 1 January 2400 fall on the same weekday. Within that cycle, the leap year rule prevents any simpler repetition.
One consequence of the cycle is that the 13th of the month falls on a Friday slightly more often than on any other weekday. It is a genuine statistical artefact of the leap year rule rather than superstition having any influence, and it happens 688 times in every 4,800 months.
Things worth knowing
- The Gregorian weekday pattern repeats exactly every 400 years.
- Dates before the Gregorian reform of 1582 need care, since countries adopted it at very different times — Britain in 1752, Russia in 1918.
- ISO 8601 treats Monday as the first day of the week; US convention uses Sunday.
- Doomsday algorithms let practised people compute weekdays mentally in a few seconds.
- The 13th falls on a Friday marginally more often than on any other day, a consequence of the leap year cycle.
Frequently asked questions
What day of the week was I born?+
Enter your birth date and the calculator will tell you. It uses Gregorian calendar arithmetic, valid for any date after your country adopted that calendar.
How can I work out a weekday without a calculator?+
The Doomsday algorithm assigns an anchor weekday to each year, from which nearby dates can be derived mentally. With practice it takes a few seconds.
Does the weekday pattern repeat?+
Yes, exactly every 400 years, because the Gregorian cycle contains 146,097 days and that number is divisible by seven. Shorter repetitions are broken by the leap year rule.
Do historical dates work correctly?+
For dates after your country adopted the Gregorian calendar, yes. Earlier dates were recorded in the Julian calendar, and adoption ranged from 1582 to 1918 depending on the country.