Day of the Week Calculator
Find out what day of the week any past or future date falls on.
Find out what day of the week any date falls on — past, present, or future.
How to Find the Day of the Week for Any Date
Zeller's Congruence
The classic algorithm for computing the day of the week was published by Christian Zeller in 1882. It maps any Gregorian or Julian calendar date to a day number using modular arithmetic — no lookup tables required.
The Gregorian Calendar
The Gregorian calendar, introduced in 1582, fixed the Julian calendar's drift by skipping leap years on century years not divisible by 400. This 400-year cycle repeats exactly, so the day-of-week pattern recurs every 400 years. The calendar shows this pattern with UK and US public holidays highlighted.
Doomsday Rule
Mathematician John Conway's Doomsday Rule lets people calculate the day of the week mentally. It relies on "anchor days" — dates in each year that always share the same day — to work out the offset for any date quickly.
Historical Dates
For dates before 15 October 1582 the Julian calendar was in use, so calculated days may differ from historical records. JavaScript's Date object uses the proleptic Gregorian calendar, projecting Gregorian rules backward. For counting the days between two historical dates, try the date calculator.
Worked example
Enter 4 July 2026 → Thursday (US Independence Day falls midweek in 2026). Enter 1 January 2000 → Saturday (Y2K weekend began on a Saturday). Enter 6 June 1944 → Tuesday (D-Day was a Tuesday). The algorithm handles any Gregorian date, including all leap years, correctly.
Related calculations
To count the days between two dates rather than name one, use the date calculator. If weekends and bank holidays need excluding, the business days calculator does that for each UK nation.
Frequently Asked Questions
What day of the week was a date in the past?
Enter any date in the calculator and it will instantly tell you the day of the week. The algorithm works for any date in the Gregorian calendar, correctly handling leap years and the 400-year Gregorian cycle. Dates before 15 October 1582 use the proleptic Gregorian calendar.
How does the day-of-the-week algorithm work?
The classic algorithm is Zeller's congruence (1882). It maps any Gregorian date to a day number using modular arithmetic and specific constants for month and year adjustments — no lookup tables needed. The formula directly computes the day from the date's numerical components.
Is the calculation accurate for very old historical dates?
For dates before 15 October 1582, the Julian calendar was in use. JavaScript's Date object projects Gregorian rules backwards (proleptic Gregorian), which may not match historical records for pre-1582 dates. For all dates from 1582 onwards, the result is fully accurate.
What is the Doomsday rule?
The Doomsday rule is a mental calculation method developed by mathematician John Conway. It uses 'anchor days' — dates that always fall on the same weekday within a year — to derive the day for any other date by counting forward or backward. Mastering it lets you calculate any date's weekday in your head.
Evidence & Methodology
How This Page Is Grounded
Method
Converts the date to a Gregorian calendar day index and maps that index to the weekday.
Important limitation: Historical results use the proleptic Gregorian calendar rather than every locality's historical calendar adoption date.
Primary Sources
- ECMAScript Date Objects Ecma International / TC39
- ISO 8601 date and time format International Organization for Standardization
Quality Checks
Known reference dates, leap years and century boundaries are checked.
See how sources are selected and corrections are handled in our Editorial & Calculation Methodology, and which automated checks this page has to pass in How We Test.