Working it out in your head
John Conway's Doomsday algorithm lets you find any weekday mentally in a few seconds once practised. It rests on a neat observation: several easily remembered dates in every year all fall on the same weekday, called that year's "doomsday".
The doomsday dates
| Month | Doomsday date | Memory aid |
|---|---|---|
| January | 3rd (4th in leap years) | 3 for 3 years in 4 |
| February | 28th (29th in leap years) | The last day of the month |
| March | 14th | Pi day |
| April | 4th | 4/4 |
| May | 9th | 9-to-5 at 7-11 |
| June | 6th | 6/6 |
| July | 11th | 9-to-5 at 7-11 |
| August | 8th | 8/8 |
| September | 5th | 9-to-5 at 7-11 |
| October | 10th | 10/10 |
| November | 7th | 9-to-5 at 7-11 |
| December | 12th | 12/12 |
The steps
- Find the century anchor. 1900s = Wednesday, 2000s = Tuesday, 2100s = Sunday.
- Take the last two digits of the year. Divide by 12; note the quotient and remainder; divide the remainder by 4.
- Add all three to the century anchor, mod 7. That is the year's doomsday.
- Count forward or back from the nearest doomsday date in your target month.
Calendar patterns worth knowing
- The Gregorian calendar repeats every 400 years exactly. 2026 has the same day-date pattern as 1626 and 2426.
- The 13th falls on a Friday slightly more often than any other weekday. Over a 400-year cycle it happens 688 times, against 684–687 for other days.
- Every year contains at least one Friday the 13th, and at most three.
- A common year shifts weekdays by one and a leap year by two, which is why your birthday moves forward by a day most years and two after a February leap day.
- The Gregorian calendar began on 15 October 1582, immediately after 4 October in the Julian calendar. Ten days simply never existed. Britain and its colonies did not adopt it until 1752, dropping eleven days.
ISO weeks and day of year
The ISO 8601 week number runs 1 to 53, with weeks starting on Monday. Week 1 is the week containing the first Thursday of the year — equivalently, the week containing 4 January. This means 1 January can fall in week 52 or 53 of the previous year.
The day of year counts from 1 January as day 1, running to 365 or 366. It is widely used in scientific data, aviation and logistics because it avoids month-length ambiguity entirely.
To count days between two dates, use the days between dates calculator. To project forward from a date, the date calculator.
Frequently asked questions
What day of the week was I born?
Enter your date of birth above. The calculator handles leap years and works for any date from the year 1 onwards.
For a mental method, the Doomsday algorithm described above gets you there in a few seconds with practice.
Why does my birthday move to a different day each year?
A common year is 365 days, which is 52 weeks plus one day, so weekdays shift forward by one. A leap year adds two days of shift.
That is why your birthday advances one weekday most years, and jumps two after a 29 February has passed.
How often does Friday the 13th occur?
At least once and at most three times a year. Over the 400-year Gregorian cycle, the 13th falls on a Friday 688 times — marginally more often than on any other weekday.
This is a quirk of the leap-year rules, not superstition.
What is an ISO week number?
A week number from 1 to 53 where weeks begin on Monday and week 1 contains the year's first Thursday.
Because of that rule, 1 January can fall in the last week of the previous year, and some years have 53 weeks.