What Day Was I Born?

Enter a birth date to see the day of the week it fell on. July 20, 1969 was a Sunday. The steps use John Conway’s Doomsday method, a trick for finding the weekday of any date in your head.

How to use the calculator

1. Choose the month and type the day and year.
2. Press Find the day.
3. You get the weekday, the month’s calendar, the Doomsday steps, and (when the date is in the past) how many days you have been alive and when your next birthday falls.

The Doomsday method

Every year, certain easy dates fall on the same weekday, called the year’s “doomsday”: 4/4, 6/6, 8/8, 10/10, 12/12, 5/9, 9/5, 7/11, 11/7, the last day of February, and January 3 (January 4 in leap years).

1. Century anchor: 1800s Friday, 1900s Wednesday, 2000s Tuesday, 2100s Sunday.
2. Take the last two digits y. Add ⌊y/12⌋, y mod 12, and ⌊(y mod 12)/4⌋ to the anchor, mod 7.
3. Count from the nearest doomsday date to your date.

Example 1969: anchor Wednesday (3); 69 = 5 × 12 + 9; 9 ÷ 4 = 2; 3 + 5 + 9 + 2 = 19 ≡ 5, Friday. July 11 is a Friday, so July 20 (9 days later) is a Sunday.

Leap years

A year is a leap year if it is divisible by 4, except century years, which must be divisible by 400. So 2000 was a leap year but 1900 was not. The calculator uses the Gregorian calendar, which began in 1582.

The old rhyme

A traditional English nursery rhyme gives each weekday a trait, starting “Monday’s child is fair of face.” It is folklore, of course; the calculator shows your line for fun.

Calendar math practice quiz: 10 questions

Test yourself with 10 multiple-choice questions. Choose an answer and press Submit to see if you are right and why. Your score appears at the end.

Frequently asked questions

How can I find out what day I was born?

Enter your birth date above. The calculator gives the weekday and shows how to work it out by hand with the Doomsday method.

What day of the week was January 1, 2000?

A Saturday.

Why does my birthday move by one weekday each year?

A regular year has 365 days, which is 52 weeks and 1 day. After a February 29, it moves by 2.

Does it work for dates before 1583?

No. Different places switched from the Julian to the Gregorian calendar at different times, so earlier weekdays depend on the calendar used.

What is my 10,000th day?

The calculator shows it when you enter a past birth date. It is about 27 years and 4 months after birth.

Sources

• Wolfram MathWorld: Doomsday Algorithm
• U.S. Naval Observatory: Leap Years

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