Day of Week Calculator
Find the weekday for any date, including birth dates and future dates, with ISO day number, ISO week-year, day of year, weekend status and leap-year context.
Find the weekday for any date
The dominant English intent is direct: enter a calendar date and answer which day of the week it was, is or will be. Birth-date lookups are a common use case.
The implementation uses calendar date functions; classic algorithms such as Zeller's congruence and the Doomsday rule are alternative ways to reach the same weekday for the same calendar system.
ISO week and calendar context
- Weekday name and ISO weekday number.
- ISO week-year for dates near New Year boundaries.
- Day of year and days remaining in the year.
- Weekend or weekday status without holiday calendars.
- Leap-year handling, including February 29 comparison notes.
Frequently Asked Questions
Sources and References
- DateMDN Web Docs
- Representing dates and timesMDN Web Docs
- Gregorian calendarBritannica
- ISO 8601 date and time formatISO
- International System of Units: TimeNIST
Calculations are based on the listed reference sources. Links open in a new tab.
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