There's a clever formula that tells you which day of the week any date falls on. With practice you can do birthdays in your head.
Quick check
In the Doomsday rule, 4/4, 6/6, 8/8, 10/10 and 12/12 all fall on the same weekday in any year. What is that anchor day called?
The rough recipe
- Take the year's last two digits.
- Divide by 4, drop the remainder, add it back on.
- Add the day of the month and a month code.
- Add the century code, then take the result mod 7 — 0 = Sunday, 1 = Monday, …
John Conway's Doomsday rule is the streamlined version. Practised mental calculators name the weekday of any date in under two seconds, no paper.
Your turn
Why do we take 'mod 7' at the end?
The trick (rough)
Take the year's last two digits, divide by 4 (drop remainder), add the day, add a code for the month, add a code for the century. Mod 7. Map 0=Sunday, 1=Monday, …
John Conway's 'Doomsday rule' is a faster version. Skilled practitioners can do any date in under 2 seconds — purely in their head.