Give this calculator any date and it returns the weekday, along with where that date sits in its year. It works for dates centuries in the past or future because it counts real days rather than looking anything up in a table.
The Gregorian calendar repeats on a 400-year cycle: 146,097 days, which is exactly 20,871 weeks. That is why 1 January 2000 and 1 January 2400 are both Saturdays, and why the same date falls on the same weekday every 400 years without exception.
If you want to do this in your head, the Doomsday algorithm is the practical method. Certain dates in every year always share a weekday — 4/4, 6/6, 8/8, 10/10, 12/12 and the last day of February — so once you know that year’s "doomsday" you can reach any date with a small adjustment.
The formula
N- Day number — whole days since 1 January 1970
+4- Offset that shifts the epoch Thursday to a Sunday-first index
mod 7- The remainder, giving 0 for Sunday through 6 for Saturday
How it works, step by step
- Enter any date in YYYY-MM-DD form.
- The gauge points to the weekday, numbered Monday to Sunday.
- The named weekday appears in the breakdown.
- Day of year, ISO week and the distance from today are shown alongside.
Worked examples
3 November 2026
That date is a Tuesday, day 307 of the year, in ISO week 45. Its ISO weekday number is 2.
The 400-year cycle
1 January 2000 was a Saturday, and 1 January 2400 will also be a Saturday. The Gregorian cycle is 146,097 days long, which divides by seven exactly 20,871 times with nothing left over.
How to read your score
Frequently asked questions
Does this work for dates before 1970?
Yes. The day number simply goes negative, and the arithmetic is unchanged. The calculator handles the nineteenth century as readily as the twenty-second.
What about dates before the Gregorian calendar?
The calculator uses the proleptic Gregorian calendar, meaning it projects today’s rules backwards. For dates before a country adopted the Gregorian calendar — 1752 in Britain, 1582 in much of Catholic Europe — the historical Julian weekday would have been different.
Why is Sunday sometimes 0 and sometimes 7?
JavaScript numbers weekdays from 0 for Sunday, while ISO 8601 numbers them 1 for Monday through 7 for Sunday. This page shows the ISO number on the gauge because that is the international standard.
Which weekday is most common for a given date?
Over a full 400-year Gregorian cycle the distribution is very slightly uneven. The 13th of the month falls on a Friday marginally more often than on any other weekday — 688 times out of 4,800.
Weekday reference
| Date | Weekday | Day of year | ISO week |
|---|---|---|---|
| 1 March 2023 | Wednesday | 60 | 9 |
| 1 March 2024 | Friday | 61 | 9 |
| 1 March 2025 | Saturday | 60 | 9 |
| 1 March 2026 | Sunday | 60 | 9 |
| 1 March 2027 | Monday | 60 | 9 |
| 1 March 2028 | Wednesday | 61 | 9 |
| 1 March 2029 | Thursday | 60 | 9 |
| 1 March 2030 | Friday | 60 | 9 |
A date advances one weekday per common year and two across a leap year, which is why the pattern repeats only every 28 years within a century.