What is a doomsday in a calendar?

A doomsday is a date that shares its weekday with every other doomsday of the same year. 4 April, 6 June, 8 August, 10 October and 12 December are doomsdays, and so are 9 May, 5 September, 11 July and 7 November. In 2026 they all fall on a Saturday, so knowing one of them tells you all the rest.

How do you work out the day of the week for any date?

To find the weekday of any date, start from the century’s anchor day: Wednesday for the 1900s and Tuesday for the 2000s. Add the year’s last two digits to a quarter of them, take out the sevens, and count on from the anchor. Then count from the month’s doomsday to your date.

How do you remember the doomsday dates?

Remember them in two groups. The even months pair with themselves: 4/4, 6/6, 8/8, 10/10 and 12/12. Four more swap their numbers in pairs: 9 May with 5 September, and 11 July with 7 November. Add 14 March and the last day of February, and January’s is the 3rd, or the 4th in a leap year.

Why do I get the wrong weekday for dates in January and February?

Leap years are the usual cause. In a leap year the doomsdays of January and February move to 4 January and 29 February, a day later than in other years. Remember that 2000 was a leap year but 1900 was not, because a year ending in 00 must divide by 400.

Is this calendar game free, and how would a class use it?

It is free in the browser, with every level open and no sign-up needed on school laptops or tablets. For a warm-up, write a pupil’s birthday this year on the board and let the class find its weekday together, one step at a time. The schools plan, priced per student, adds a dashboard showing who is getting quicker.

Math practice · reimagined

Day of the Week Game

In this day-of-the-week challenge a date appears, anywhere from 1 January 1900 to the last day of 2099, and you answer on seven keys that carry the names of the days. Work it out with the Doomsday method before your rival, and every right day lands a punch.

Ages 4–99+ · no download · plays in your browser · 23 languages

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What is the Doomsday method?

The Doomsday method finds the weekday of any date in your head. It leans on dates that always share a weekday in a given year, like 4 April, 6 June and 8 August, which are called doomsdays. You find this year's doomsday, pick the one in your month, and count the days between.

Every level of Calendar

Example: October 16, 2020

Years from 2000 to 2099 count from Tuesday. For 2020, add 20 and a quarter of 20, which is 5, to get 25.
Take out the sevens: 25 − 21 = 4. Four days after Tuesday is Saturday, so every doomsday in 2020 is a Saturday.
October’s doomsday is 10 October, a Saturday. The 16th is six days later, and six days after Saturday is Friday.

Learn it step by step

Some people can tell you the day of the week of any date you name: your birthday, the first day of school, a day in 2087. It looks like magic. It is a method, and you can learn it.

It starts with something odd about calendars: some dates ALWAYS share a weekday. In any year, 4 April, 6 June, 8 August, 10 October and 12 December all land on the same day of the week. So do 9 May, 5 September, 11 July and 7 November, and 14 March, and the last day of February. These dates are called the doomsdays. Know the weekday of one of them and you know them all.

Every year from 2000 to 2099 is counted from the same starting day, called the anchor, and for this century it is Tuesday. To find a year's doomsday, take its last two digits, add a quarter of them (rounded down, which counts the leap years), and take away all the sevens you can. What is left is how many days to count on from Tuesday.

Try 2026. 26 + 6 = 32. Take away the sevens (28) and 4 is left. Four days after Tuesday is Saturday, so every doomsday in 2026 is a Saturday: 4 April, 6 June, 10 October, all of them.

Now the date you were asked. Find the doomsday in its month and count from it. For 25 December 2026: 12 December is a Saturday, and the 25th is 13 days later. Take away a week and 6 days are left, and six days after Saturday is Friday. Christmas 2026 is a Friday. Counting backwards works the same way: 1 April 2026 is three days before the 4th, so it is a Wednesday.

One warning about January and February. In a leap year the doomsday is 4 January and 29 February; in other years it is 3 January and 28 February. A year is a leap year when 4 divides it, and 2000 counts.

Find the year's doomsday, find the month's, count, and press the day your count lands on.

Tips and common mistakes

Learn the weekday of this year’s doomsday once, in January. For the rest of the year, any date is a short count away from 4/4, 6/6, 8/8, 10/10 or 12/12.
A common slip is counting the doomsday itself. From Saturday the 10th, the 11th is one day on, not two, so count the steps between dates.
Ask your child for the weekday of a family birthday, then check it on a paper calendar together. A right answer feels like a magic trick.

How to play

A date appears for both fighters, written out in full, and seven keys below show the days of the week. Press the day you worked out: the first right key lands the hit for Chilli or your rival, and a faster answer hits harder. A wrong day stuns you for a moment.

— Quick answers —

Calendar: the short FAQ

What is a doomsday in a calendar?: A doomsday is a date that shares its weekday with every other doomsday of the same year. 4 April, 6 June, 8 August, 10 October and 12 December are doomsdays, and so are 9 May, 5 September, 11 July and 7 November. In 2026 they all fall on a Saturday, so knowing one of them tells you all the rest.
How do you work out the day of the week for any date?: To find the weekday of any date, start from the century’s anchor day: Wednesday for the 1900s and Tuesday for the 2000s. Add the year’s last two digits to a quarter of them, take out the sevens, and count on from the anchor. Then count from the month’s doomsday to your date.
How do you remember the doomsday dates?: Remember them in two groups. The even months pair with themselves: 4/4, 6/6, 8/8, 10/10 and 12/12. Four more swap their numbers in pairs: 9 May with 5 September, and 11 July with 7 November. Add 14 March and the last day of February, and January’s is the 3rd, or the 4th in a leap year.
Why do I get the wrong weekday for dates in January and February?: Leap years are the usual cause. In a leap year the doomsdays of January and February move to 4 January and 29 February, a day later than in other years. Remember that 2000 was a leap year but 1900 was not, because a year ending in 00 must divide by 400.
Is this calendar game free, and how would a class use it?: It is free in the browser, with every level open and no sign-up needed on school laptops or tablets. For a warm-up, write a pupil’s birthday this year on the board and let the class find its weekday together, one step at a time. The schools plan, priced per student, adds a dashboard showing who is getting quicker.

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