Pick any date to see the trick worked out, step by step
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Cheat sheet
Doomsday dates: 4/4, 6/6, 8/8, 10/10, 12/12, and "9-to-5 at 7-Eleven" — 9/5, 5/9, 7/11, 11/7. Plus 3/14 (Pi Day), and 3 Jan / 28 Feb, which become 4 Jan / 29 Feb in a leap year.
Century anchors: 1600s and 2000s Tuesday · 1700s and 2100s Sunday · 1800s and 2200s Friday · 1900s and 2300s Wednesday.
Year: take the last two digits, add the whole 12s, the leftover, and the leap years inside the leftover.
How the trick works
Every year has one weekday — its doomsday — that a handful of easy dates all land on. Learn those dates, work out the doomsday for the year, and then count a few days forward or back.
1️⃣ Memorize the doomsday dates
These fall on the same weekday in any given year. The even months pair with themselves; the odd ones are the "9-to-5 at 7-Eleven" pairs:
4/4 6/6 8/8 10/10 12/12 and 9/5 5/9 7/11 11/7
March is 3/14, Pi Day. January and February are the odd ones out: 3 January and 28 February normally, but 4 January and 29 February in a leap year — so the last day of February is always a doomsday.
2️⃣ Learn the four century anchors
The pattern repeats every 400 years, so four weekdays cover every century:
| 1600s, 2000s, 2400s | Tuesday |
| 1700s, 2100s | Sunday |
| 1800s, 2200s | Friday |
| 1900s, 2300s | Wednesday |
If you'd rather compute it than remember it: (5 × (century mod 4) + 2) mod 7, counting Sunday as 0.
3️⃣ Step from the anchor to the year
Take the last two digits of the year, then add three numbers to the anchor: how many whole 12s fit, what's left over, and how many leap years hide in that leftover.
1987 → 87 = 7×12 + 3, and 3 ÷ 4 = 0 → 7 + 3 + 0 = 10
The 1900s anchor is Wednesday, and 10 days past Wednesday is Saturday. So every doomsday date in 1987 — 4 April, 6 June, 9 May — is a Saturday.
➗ Another way to do step 3: odd + 11
Same result, different arithmetic — two halvings instead of three additions. Some people are faster this way; try both and keep the one your head likes.
Take the last two digits, then: if it's odd, add 11. Halve it. If that's odd, add 11. Finally subtract what's left over from the next multiple of 7.
87 → +11 = 98 → ÷2 = 49 → +11 = 60 → 7 − (60 mod 7) = 3
Wednesday + 3 is Saturday — the same 1987 doomsday the 12s method found. The two offsets, 10 and 3, differ by exactly a week, so they always land on the same day. Learn mode shows both rows side by side for whatever date you pick.
4️⃣ Count to your date
Find the doomsday date in the same month and count the difference, dropping whole weeks as you go.
12 June 1987: 6 June is a Saturday, +6 days → Friday
🕰️ One caveat about old dates
This is the Gregorian calendar, and countries adopted it at different times — Britain in 1752, Sweden in 1753. For a date before then, the answer is what the Gregorian calendar would have said, not what the almanac of the day printed.