Figure out the day of the week in your head.

Pick any date. Learn a simple method to tell whether it lands on Monday, Tuesday, or any other day of the week using mental math.

Step 1

Pick a date.

Choose one date. We will use the Doomsday algorithm to find its weekday together.

Month

Day

Year

Step 2

The first number comes from the century. For this tutorial, use the chart below.

You need to memorize this if you want to do the calculation in your head: the 1900s start with Wednesday = 3, and the 2000s start with Tuesday = 2.

Years	Start with	Number
1900–1999	Wednesday	3
2000–2099	Tuesday	2

Sun
0Mon
1Tue
2Wed
3Thu
4Fri
5Sat
6

Step 3

Use the last two digits of the year. Break them into small pieces, then add the century number.

Step 4

Anchor dates are shortcuts. Every anchor date lands on the year’s Doomsday weekday.

You need to memorize these anchor dates if you want to do the calculation in your head. January and February change in a leap year.

Step 5

Count forward or backward from the anchor date. Remove full groups of seven because weekdays repeat every seven days.

Sun
0Mon
1Tue
2Wed
3Thu
4Fri
5Sat
6

Step 6

That is the complete Doomsday algorithm: century number, year math, anchor date, then count the offset.

Study Mode

Review the facts you need if you want to do the Doomsday calculation in your head.

Start with these three reference cards: century numbers, weekday numbers, and anchor dates.

Years	Start with	Number
1900–1999	Wednesday	3
2000–2099	Tuesday	2

Sun
0Mon
1Tue
2Wed
3Thu
4Fri
5Sat
6

Leap years: use Jan 4 and Feb 29.

Quiz Mode

Solve this like the lesson: century number, year math, month anchor, then count from the anchor.

Use this summary to spot where your mental math may have drifted.