Day of the Week Calculator

How to use the weekday calculator

Choose a date with the date picker (or type it as year-month-day) and press the button. The tool answers with the name of the weekday. It is pre-filled with today's date, so one click tells you what day it is.

Typical uses: finding the weekday you were born on, checking that a planned event does not land on a weekend, or confirming what day a historical date fell on. To see the whole month around the date, open the calendar calculator. To jump forwards or backwards by a number of days and then read the weekday, use the add days calculator.

How a weekday is worked out

The weekdays repeat in a strict cycle of seven, so the task is to count how many days separate your date from a known one and keep the remainder after dividing by 7. Zeller's congruence packs that counting into a single formula for the Gregorian calendar:

h = (q + ⌊13 × (m + 1) ÷ 5⌋ + K + ⌊K ÷ 4⌋ + ⌊J ÷ 4⌋ + 5 × J) mod 7

• q is the day of the month.
• m is the month counted from March = 3 to December = 12. January and February count as months 13 and 14 of the previous year.
• K is the last two digits of that year, and J is the first two digits (the century).
• h is the weekday: 0 = Saturday, 1 = Sunday, 2 = Monday, 3 = Tuesday, 4 = Wednesday, 5 = Thursday, 6 = Friday.

Example, 20 July 1969: q = 20, m = 7, K = 69, J = 19. The terms are 20 + ⌊13 × 8 ÷ 5⌋ + 69 + ⌊69 ÷ 4⌋ + ⌊19 ÷ 4⌋ + 5 × 19 = 20 + 20 + 69 + 17 + 4 + 95 = 225. Since 225 = 32 × 7 + 1, h = 1, which is a Sunday.

Example, 1 January 2000: January counts as month 13 of 1999, so q = 1, m = 13, K = 99, J = 19. The sum is 1 + 36 + 99 + 24 + 4 + 95 = 259, and 259 = 37 × 7 exactly, so h = 0, a Saturday.

Why the weekday of a date keeps changing

A common year has 365 days, which is 52 weeks and 1 day, so a given calendar date falls one weekday later each year. When a 29 February sits between the two dates the shift is two weekdays. For instance, 15 June 2023 was a Thursday, 15 June 2024 a Saturday (the 29 February 2024 in between adds the extra day) and 15 June 2025 a Sunday.

Over a full 400-year Gregorian cycle there are 146,097 days, which is exactly 20,871 weeks. The whole pattern of weekdays therefore repeats every 400 years: 1 January 2000 and 1 January 2400 are both Saturdays. That regularity is a consequence of the leap year rule.

Which day does the week start on?

The seven-day week is shared almost everywhere, but countries disagree about where it begins. The international standard ISO 8601 starts the week on Monday, and so does most of Europe. The United States, Canada and a number of other countries start it on Sunday, and some Middle Eastern countries start on Saturday. The weekday calculator does not depend on this choice, because it only names the day.

Which days are the weekend

To count only the days that people actually work between two dates, see the business days calculator, and for the plain number of days in between use the days between dates calculator.

Limits for very old dates

The calculator applies the Gregorian calendar to every date, even those before it was introduced in October 1582 (this is called the proleptic Gregorian calendar). Documents from earlier periods were usually written in the Julian calendar, whose dates sit several days apart from the Gregorian ones, so a historical weekday found here can disagree with the original record. Enter years with four digits. A day that does not exist, such as 30 February, rolls forward into the next month (30 February 2026 is read as 2 March 2026).