How to use the doomsday calculator
Pick any date from 1583 to 9999. The result shows the weekday, the day of the year, the ISO week number and whether the year is a leap year. Below it, the Doomsday rule table goes through the calculation one step at a time: the century anchor day, the year’s doomsday, the nearest doomsday in the month and the count to your date. The same tool is on the day of the week calculator page.
The idea behind the Doomsday rule
In any year, a set of easy-to-remember dates all fall on the same weekday, called that year’s doomsday. Once you know the doomsday, you find the anchor date closest to your date and count forward or back.
| Month | Doomsday date | How to remember it |
|---|---|---|
| January | 3 (common year) or 4 (leap year) | 3 in three years, 4 in the fourth |
| February | 28 (common year) or 29 (leap year) | The last day of February |
| March | 14 | Pi Day (3/14); also “March 0”, the last day of February |
| April, June, August, October, December | 4 April, 6 June, 8 August, 10 October, 12 December | Even months: day equals month (4/4, 6/6 and so on) |
| May and September | 9 May and 5 September | “I work 9 to 5 at the 7-Eleven” (5/9 and 9/5) |
| July and November | 11 July and 7 November | The same phrase (7/11 and 11/7) |
Step 1: the century anchor
Each century has an anchor day, which repeats every 400 years:
| Years | Anchor day |
|---|---|
| 1800–1899 | Friday |
| 1900–1999 | Wednesday |
| 2000–2099 | Tuesday |
| 2100–2199 | Sunday |
The 1600s share the 2000s anchor (Tuesday) and the 1700s share the 2100s anchor (Sunday).
Step 2: the year’s doomsday (the 12s method)
The calculator uses the last two digits of the year, y:
- Divide y by 12: the whole-number result is a, the remainder is b.
- Divide b by 4, dropping any fraction: that is c.
- Add a + b + c, and add that many days to the century anchor. Only the remainder after dividing by 7 matters.
For 2026: 26 = 2 × 12 + 2, so a = 2, b = 2 and c = 0. The sum is 4, and Tuesday + 4 days is Saturday, the doomsday of 2026.
Worked example: 20 July 1969
- The century anchor for the 1900s is Wednesday.
- 69 = 5 × 12 + 9, so a = 5 and b = 9. Then 9 ÷ 4 = 2 (dropping the remainder), so c = 2.
- Counting Sunday as 0, Wednesday is 3. Then 3 + 5 + 9 + 2 = 19, and 19 mod 7 = 5, which is Friday. Friday is the doomsday of 1969.
- In July the doomsday date is 11 July (7/11), so 11 July 1969 was a Friday.
- 20 July is 9 days later. Friday + 9 days = Friday + 2 days = Sunday.
So 20 July 1969, the day of the Apollo 11 Moon landing, was a Sunday.
A second check: 25 December 2026. The doomsday is Saturday, 12 December is a doomsday, and 25 December is 13 days later. Saturday + 13 days = Saturday + 6 days = Friday.
The rule works because the Gregorian calendar repeats every 400 years. To learn more about leap years, which shift the January and February anchors, see what is a leap year, and for the names of the weekdays themselves see the origin of the days of the week.