How to Find the Day of the Week for Any Date

Why weekdays are predictable

The days of the week repeat in a cycle of seven, and the calendar repeats in a pattern that depends on leap years. That regularity means that you can work out the weekday of any date with a little arithmetic and no lookup.

The weekday of a date moves forward by one from year to year, and by two across a leap day. The same date in a series of years can be followed easily: 22 October falls on a Thursday in 2026, a Friday in 2027 and a Sunday in 2028.

Within a month, dates seven days apart share a weekday. If you know that 22 October 2026 is a Thursday, then 1 October, 8 October and 15 October, 29 October, are Thursdays too.

The doomsday method

The mathematician John Conway popularised a mental method based on one idea: in every year, a set of easy-to-remember dates falls on the same weekday. This weekday is called the doomsday of the year. The dates are 4 April, 6 June, 8 August, 10 October and 12 December, which are the even months written as pairs, together with 9 May and 5 September, 11 July and 7 November, which are easy to remember as 9-to-5 at work and 7-Eleven.

The last day of February, the 28th in an ordinary year and the 29th in a leap year, is also a doomsday. In January the doomsday is the 3rd in an ordinary year and the 4th in a leap year.

Once you know the doomsday of the year, you find the weekday of any date by counting from the nearest doomsday in that month.

Finding the doomsday of a year

Each century has an anchor weekday. For the years 2000 to 2099, it is Tuesday, and for 1900 to 1999 it is Wednesday. Take the last two digits of the year and call them y. Divide y by 12 and write down the whole number. Take the remainder and divide it by 4, again ignoring the fraction. Add the two results to the remainder and to the anchor weekday, counting Sunday as 0.

For 2026: y is 26. 26 divided by 12 is 2 with a remainder of 2. The remainder divided by 4 is 0. The sum is 2 + 2 + 0 = 4. Tuesday plus 4 days is Saturday, so the doomsday of 2026 is Saturday. You can confirm it: 28 February 2026 was a Saturday.

For 2027: y is 27, which gives 2 with a remainder of 3, then 0, and the sum is 5. Tuesday plus 5 is Sunday. In 2027 the dates 4 April, 6 June and 10 October are Sundays.

A worked example: 22 October 2026

The doomsday of 2026 is Saturday, and 10 October is a doomsday, so 10 October 2026 is a Saturday. From 10 to 22 is 12 days. Divide by 7: the remainder is 5. Five days after Saturday is Thursday, so 22 October 2026 is a Thursday.

Try another: 25 December 2026. From 12 December, a Saturday, to 25 is 13 days, a remainder of 6. Six days after Saturday is Friday. 25 December 2026 is a Friday.

With a little practice, the method takes about as long as reading the question. It is a good party trick, and it helps you to see why calendars repeat.

Famous dates for practice

• 1 January 2000 was a Saturday.
• 20 July 1969, the first Moon landing, was a Sunday.
• 9 November 1989, the fall of the Berlin Wall, was a Thursday.
• 29 February 2028 will be a Tuesday.

Use these to test the method or the calculator. Calendars repeat after 400 years, which is 146,097 days, an exact number of weeks, so the weekday of any date repeats every 400 years in the Gregorian calendar.

Limits of the method

The doomsday method works for the Gregorian calendar. For dates before it was introduced, the weekday follows the Julian calendar in the country concerned, and the date may be different from the one we would use today. The calculator on this site applies the Gregorian calendar to every year, so very old dates will not match a historical source.

The method also assumes that you know whether the year is a leap year. If you are not sure, apply the leap-year rule first.

Checking the method against a calendar

The best way to learn the doomsday method is to test it on dates you already know. Start with your own birthday this year, work out the weekday, and compare it with a calendar. If you are one day out, the usual cause is the century anchor or forgetting that January and February dates in a leap year use a different doomsday.

Then try dates in other years. Each time you move the year forward by one, the doomsday moves one weekday forward, or two after a leap year, and that pattern is enough to reach any year in a few steps. Within a few attempts you will find that the whole procedure fits in your head.

Using the calculator

Open the Day of the week tab and enter the date. The result shows the weekday in large type, with the ISO week number, the day of the year, the days left in the year, the days in the month and the quarter.

Use the calculator to check your mental result. If they match, the method is working, and the more often you try, the faster it gets.