Day of Week Calculator
Any calendar date falls on exactly one day of the week, fixed by the Gregorian calendar's regular structure. This calculator takes a date and returns the named weekday, the ISO 8601 weekday number (Monday is 1 through Sunday is 7), and the day of the year. It applies the Gregorian leap-year rule so 29 February and post-February day counts are exact. Use it to plan events, check what day a birthday or deadline lands on, settle a bet, or convert dates into the ISO weekday numbers that scheduling systems expect.
Day of week logic
Weekday index = days from a known reference date, mod 7
JavaScript getUTCDay: Sunday = 0 ... Saturday = 6
ISO weekday = ((getUTCDay + 6) mod 7) + 1 (Monday = 1 ... Sunday = 7)
Day of year = cumulative days of prior months + day, with leap adjustment
The date engine implements proleptic Gregorian rules. ISO 8601 numbers Monday first, so the calculator converts from the Sunday-first index returned by the engine.
Calendar context
- The Gregorian calendar repeats exactly every 400 years (146,097 days), so weekday patterns are fully predictable.
- ISO 8601 defines Monday as the first day of the week, numbered 1, and Sunday as 7.
- A common year has 365 days; a leap year has 366, adding 29 February.
- Leap years are divisible by 4, except centuries, which must be divisible by 400 (so 1900 was not a leap year; 2000 was).
- Before 1582 this tool uses the proleptic Gregorian calendar, which can differ from the Julian dates in historical use.
Day of week: frequently asked questions
How is the day of the week determined?
Every calendar date maps to a fixed day of the week because the Gregorian calendar repeats on a known 400-year cycle. The day can be found with Zeller's congruence or, equivalently, by counting days from a reference date of known weekday. This calculator uses the calendar arithmetic built into the date engine, which implements the same Gregorian rules.
What is the ISO weekday number?
ISO 8601 numbers the days Monday = 1 through Sunday = 7. This differs from the common Sunday = 0 convention used in some programming contexts. The calculator reports the ISO number alongside the named day to avoid ambiguity.
Does it handle leap years correctly?
Yes. The Gregorian leap-year rule (divisible by 4, except centuries not divisible by 400) is applied, so 29 February dates and the day-of-year count after February are correct for leap and common years alike.
What date range is supported?
The calculator works for any Gregorian date the date engine accepts, covering modern years comfortably. For pre-1582 dates the proleptic Gregorian calendar is used, which differs from the Julian calendar then in civil use, so treat very old dates with care.
Why might a historical date show an unexpected day?
Because many countries adopted the Gregorian calendar at different times between 1582 and the 20th century. This tool always uses the Gregorian rule (proleptic before 1582), so dates from a period when a Julian calendar was in civil use may not match historical records.
Official sources
- U.S. Naval Observatory: Astronomical Applications Department.
- National Institute of Standards and Technology: Time and Frequency Division.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.