Doomsday Calculator

Learn the Doomsday Rule

The Doomsday Rule is an algorithm created by John Conway for calculating the day of the week of any date.

Today is

5 March 2026

Click to reveal the weekday, then verify it with the method below.

The method

Learn the mental model in four short steps. The numbers below update live with your selected date.

Step 1

Century anchor

Step 2

Year doomsday

Step 3

Month method

Step 4

Leap adjustment

Weekday index reference

IndexDay
0Sunday
1Monday
2Tuesday
3Wednesday
4Thursday
5Friday
6Saturday
Step 1: Century anchor

Formula

(5 × (c % 4)) + 2

Century (c): 20

(c % 4): 0

5 × (c % 4): 0

Mod 7: 0

Add 2: 2

Anchor = 2

Step 2: Year doomsday (odd + 11)

Last two digits: 26

odd? +11 → /2 → odd? +11 → mod 7

After first odd check: 26

Divide by 2: 13

After second odd check: 24

Mod 7: 3

Century anchor + (7 - mod): 6

Doomsday index: 6 (Saturday)

Step 3: Month method

My personal shortcut: memorize one neg value per month.

(day + monthNeg) % 7

Month neg for March: 0

Day + neg: 5

Offset mod 7: 5

Final index: 4

Standard approach: memorize fixed doomsday dates (4/4, 6/6, 8/8, 10/10, 12/12, etc.). Different schemes are fine as long as offsets stay consistent mod 7.

Month neg table

MonthNeg
1 January4
2 February0
3 March0
4 April3
5 May5
6 June1
7 July3
8 August6
9 September2
10 October4
11 November0
12 December2

Standard doomsday dates: 1/3, 2/28, 3/14, 4/4, 5/9, 6/6, 7/11, 8/8, 9/5, 10/10, 11/7, 12/12

Step 4: Leap year adjustment
Leap year? subtract 1 for Jan/Feb

2026 is not a leap year.

Adjustment: no change

Worked example (live)

Century20
Anchor2
Decade26
Mod3
Doomsday6
Offset5
Leap adj.0
Final index4
WeekdayThursday