Enter your date and discover famous events that happened on the same weekday.
Shared Birth-Weekdays
Enter a date. See all dates in the current year that share the same weekday.
Training
Learn the four century anchors.
Add the century anchor and the year contribution.
Use the year’s Doomsday to solve a complete date.
Exercise 1
Exercise 2
Exercise 3
The Method
Exercise 1 · 1643
Match a century to its anchor. Remember the repeating sequence:
1 · 6 · 4 · 3
Score: 0 · Streak: 0
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Which weekday and number belong to this century?
Exercise 2 · Find the Doomsday
First find the century anchor x. Then calculate the year contribution y. Your answer is their sum z: the Doomsday for that year.
x + y = z
For the last two digits a: y = (a + floor(a ÷ 4)) mod 7. If x + y is greater than 7, subtract 7.
Score: 0 · Streak: 0
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Step 1: What is x, the century anchor?
Step 2: What is z, the year’s Doomsday? Choose the result of x + y — not y.
Exercise 3 · Find Any Weekday
Work through the same two steps. If the date is not itself a Doomsday date, use the month’s known Doomsday to reach the target date.
Score: 0 · Streak: 0
Press start.
Step 1: What is x, the century anchor?
Step 2: What is z, the Doomsday for this year? Again, answer x + y, not y.
Step 3: What weekday is the complete date?
The bday method
The bday method is a simple training approach for seeing calendar patterns and finding weekdays mentally.
It is inspired by the classic Doomsday rule developed by John Horton Conway.
July 4 and October 31
always land on the same weekday in the same year.
04/04 · 06/06 · 08/08 · 10/10 · 12/12
same pattern. Same system.
Every 400 years, weekdays repeat.
June 6, 744 has the same weekday as June 6, 1944.
Exercise 1 · 1643
Learn the century anchors: Sunday 1, Friday 6, Wednesday 4, Tuesday 3.
Exercise 2 · x + y = z
x is the century anchor. For the final two year digits a, y = (a + floor(a ÷ 4)) mod 7. The result z is the year’s Doomsday. Always answer z — the sum — not y alone.
Exercise 3 · Any date
Find z first. Then use the month’s known Doomsday date to reach the weekday of the target date.
1643 · bday method