A day of week calculator tells you which day of the week any date falls on, past or future. Formula: Parse date as local midnight; weekday = Sunday…Saturday from getDay(). May 15, 1990 was a Tuesday, and January 1, 2000 was a Saturday.
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How to use the day of week calculator
- Enter the date as year-month-day; the field starts on today’s date.
- Read the day of the week, the day of the year and the ISO week number.
How the day of week calculator works
The day of week calculator reads a date and returns the weekday it falls on, using the Gregorian calendar that most of the world uses today. The week has seven days, so weekdays repeat in a cycle, but the pattern shifts because years have 365 or 366 days. A common year moves every date forward by one weekday; a leap year moves dates after February forward by two.
- Enter a date as year-month-day, for example 1990-05-15.
- The result is the day of the week for that date.
- Leap years: every year divisible by 4, except century years not divisible by 400. 2000 was a leap year; 1900 was not.
Day of week calculator examples
| Date | Day of the week |
|---|---|
| January 1, 2000 | Saturday |
| July 20, 1969 | Sunday |
| May 15, 1990 | Tuesday |
| February 29, 2024 | Thursday |
| October 21, 2026 | Wednesday |
| July 4, 1776 | Thursday |
If you were born on May 15, 1990, you were born on a Tuesday. The Apollo 11 Moon landing on July 20, 1969 was a Sunday, and January 1, 2000 was a Saturday.
Finding the day by hand
One mental method is the Doomsday rule devised by mathematician John Conway. In every year, certain easy-to-remember dates, such as 4/4, 6/6, 8/8, 10/10, 12/12 and the last day of February, all fall on the same weekday, called the year’s doomsday. In 2026 the doomsday is a Saturday, so October 10, 2026 is a Saturday and October 21 is 11 days later, a Wednesday. Learning the rule for finding each year’s doomsday takes practice; the day of week calculator gives the answer instantly.
Zeller’s congruence
A formula published by Christian Zeller in the 19th century computes the weekday directly. Treat January and February as months 13 and 14 of the previous year, then compute h = (q + ⌊13(m + 1)/5⌋ + K + ⌊K/4⌋ + ⌊J/4⌋ + 5J) mod 7, where q is the day, m the month, K the year within the century and J the century. h = 0 means Saturday, 1 Sunday, and so on. For May 15, 1990: q = 15, m = 5, K = 90, J = 19, giving h = 3, a Tuesday.
Historic dates and calendar changes
The Gregorian calendar replaced the Julian calendar in 1582 in parts of Europe, but Britain and its colonies, including those in America, switched only in 1752, when 11 days were dropped. The day of week calculator applies the Gregorian calendar to all dates. For events recorded in the old Julian calendar, such as George Washington’s birth on February 11, 1731 (Old Style), convert the date to the Gregorian calendar first; his birthday is now given as February 22, 1732.
Everyday uses
- Find the weekday you were born, or what day your birthday falls on this year.
- Check whether a deadline, holiday or event falls on a weekend.
- Plan recurring meetings and check the weekday of a date far in the future.
Along with the weekday, the day of week calculator shows the day of the year (October 21, 2026 is day 294) and the ISO 8601 week number (2026-W43). ISO weeks start on Monday, and week 1 is the week containing the year’s first Thursday.
Repeating calendars
Because weekdays shift by one or two days each year, whole calendars repeat. The 2026 calendar, a common year starting on a Thursday, matches the calendars of 2015 and 2037, so an old 2015 wall calendar would show the right weekdays for every date this year. Leap-year calendars repeat less often, usually every 28 years. The full Gregorian cycle is exactly 400 years: those years contain 146,097 days, which is precisely 20,871 weeks, so any date has the same weekday as the date 400 years earlier.
These cycles are handy for checking results. If the day of week calculator says a date in 2026 is a Monday, the same date in 2015 and 2037 should be a Monday too. Birthdays move forward one weekday each year, or two after February 29, which is why a birthday that falls on a Friday this year will usually fall on a Saturday next year.
Related converters and calculators
Related calculators:
- Age calculator: age
- Date difference calculator: date gaps
- Time duration calculator: durations
- Time zone calculator: time zones
Sources
U.S. Naval Observatory: Julian date converter; NIST: Time and Frequency Division.
Formula and method
Parse date as local midnight; weekday = Sunday…Saturday from getDay().
Proleptic Gregorian calendar weekday from the local Date object, plus day of year and ISO week (weeks start Monday; week 1 contains the first Thursday).
Questions about this calculator
What day of the week was I born?
Enter your birth date in the tool to see the weekday.
What day was January 1, 2000?
A Saturday.
How are leap years decided?
Divisible by 4, except century years not divisible by 400.
Does it work for very old dates?
It uses the Gregorian calendar for all dates; convert Julian dates first.
What is the Doomsday rule?
A mental method by John Conway based on dates that share a weekday each year.
Check the assumptions and units before relying on a result. Financial, tax and health estimates do not replace advice for your individual circumstances.
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