Day of the Week Calculator

What the calculator shows

Enter any date and the day of the week calculator tells you which weekday it falls on. Alongside the answer you get:

• The full date, written out, such as “Friday, December 25, 2026”.
• Day of the year, for example day 359 of 365.
• ISO week number, the week number used in business calendars and planning.
• Weekday or weekend.
• The year’s doomsday, the anchor weekday used in the mental method described below.
• Whether the year is a leap year.
• Zeller’s congruence worked out with the numbers from your date.

It handles past and future dates alike, from the year 1 to 9999, so you can look up the weekday you were born on, check the day of a historical event or plan an event years ahead.

How to use it

Pick a date in the day of the week calculator, or type it. The field starts on today’s date. The result updates as soon as the date is valid. For dates before October 15, 1582, a note reminds you that the result is on the proleptic Gregorian calendar.

How to calculate the day of the week with Zeller’s congruence

Zeller’s congruence, published by Christian Zeller in the 1880s, turns a date into a weekday with whole-number arithmetic. Here it is for March 14, 1879.

1. Write down the parts: q = 14 (day), m = 3 (March), K = 79 (year of the century), J = 18 (century).
2. Month term: ⌊13 × (3 + 1) ÷ 5⌋ = ⌊52 ÷ 5⌋ = 10.
3. Year terms: K = 79, ⌊79 ÷ 4⌋ = 19, ⌊18 ÷ 4⌋ = 4 and 5 × 18 = 90.
4. Add everything: 14 + 10 + 79 + 19 + 4 + 90 = 216.
5. Take the remainder after dividing by 7: 216 mod 7 = 6.
6. Look it up: 0 = Saturday, 1 = Sunday, 2 = Monday, 3 = Tuesday, 4 = Wednesday, 5 = Thursday, 6 = Friday. March 14, 1879 was a Friday.

For a January or February date, use the previous year and add 12 to the month. February 3, 2025 becomes month 14 of 2024, so K = 24 and J = 20.

The Doomsday rule: a method you can do in your head

John Conway’s Doomsday rule relies on the fact that certain easy-to-remember dates always share a weekday within a year, called the year’s doomsday:

• 4/4, 6/6, 8/8, 10/10 and 12/12
• 5/9, 9/5, 7/11 and 11/7 (“9 to 5 at the 7-Eleven”)
• The last day of February, and March 14 (Pi Day)
• January 3 in common years, January 4 in leap years

To find a year’s doomsday, start from the century’s anchor: Friday for the 1800s, Wednesday for the 1900s, Tuesday for the 2000s and Sunday for the 2100s. Then, for the last two digits of the year:

1. Divide by 12: the quotient and the remainder.
2. Divide the remainder by 4, ignoring any fraction.
3. Add the three numbers and count that many days forward from the anchor.

For 2026: 26 ÷ 12 = 2 remainder 2, and 2 ÷ 4 = 0. Adding 2 + 2 + 0 = 4, and four days after Tuesday is Saturday. So every doomsday in 2026 is a Saturday. December 12 is a Saturday, so December 25, thirteen days later, is a Friday.

Weekdays of common dates

Notice the two-day jump between 2027 and 2028 for July 4 and December 25: the leap day, February 29, 2028, falls in between. January 1 jumps two days a year later, from 2028 to 2029.

Historic dates and the Julian calendar

The Gregorian calendar replaced the Julian calendar in parts of Europe in October 1582, when Thursday, October 4 was followed directly by Friday, October 15. Britain and its American colonies did not switch until 1752, when Wednesday, September 2 was followed by Thursday, September 14.

The day of the week calculator always uses the Gregorian calendar. It correctly gives Thursday for September 14, 1752, but for September 2, 1752 it gives Saturday, the weekday of that date on the Gregorian calendar, while people in London at the time recorded a Wednesday under the Julian calendar. George Washington was born on February 11, 1731 by the Julian calendar then in use, which is February 22, 1732 on the Gregorian calendar, a Friday.

For genealogy or history research, check which calendar a record used before reading too much into the weekday.

Real-life uses

• Finding the weekday you or a relative were born on.
• Checking whether a holiday, deadline or anniversary lands on a weekend. To count working days between two dates, use the business days calculator.
• Planning events years ahead, together with the date calculator to add or subtract days.
• Verifying dates in historical documents, letters and diaries.

Glossary

• Gregorian calendar. The calendar used in the United States today, with leap years every 4 years except century years not divisible by 400.
• Proleptic calendar. A calendar extended backward to dates before it was introduced.
• Doomsday. The weekday shared by a set of easy-to-remember dates in a given year.
• ISO week. A Monday-to-Sunday week numbered under ISO 8601.