How this calculator works
- Step 1Count daysDay numberDays since a fixed starting date, leap years included.
- Step 2Divide by 7RemainderEvery seven days the weekday repeats.
- Step 3WeekdayFridayFor 15 Aug 1947.
The calculator counts whole days from 1 January 1970, a Thursday, using the Gregorian leap-year rule. The remainder after dividing by seven gives the weekday.
Worked example: Independence Day
- Weekday
(days since a known Thursday) mod 7
- WeekdayFriday
- Day of the year227 of 365
- ISO weekWeek 33 of 1947
26 Jan 1950, when the Constitution came into force, was a Thursday. Leap day 29 Feb 2024 was a Thursday, day 60 of 366.
ISO week numbers
- 3 Jan 2021Week 53 of 2020Early January can fall in the last week of the year before.
- 30 Dec 2024Week 1 of 2025Late December can fall in week 1 of the next year.
ISO 8601 weeks start on Monday, and week 1 is the week that contains the year’s first Thursday. Some years have 53 weeks.
Read the result
The headline is the weekday; the table adds the day of the year, ISO week, quarter and whether it is a leap year.
Assumptions and limits
- Gregorian calendarUsed throughout, 1900 to 2100.
- ISO weeksMonday start; week 1 holds the first Thursday.
- No local calendarsPanchang, Hijri and other calendars are not shown.
Related date tools
The weekday is one fact about a date. Counting and shifting dates have their own tools.
To count days between two dates, use the date difference calculator. For a date some days ahead, try the add or subtract dates calculator, and for business days, the working days calculator.
Common mistakes
Mistake: Assuming week 1 always starts on 1 January.
Instead: Under ISO 8601, week 1 is the week with the first Thursday.
Mistake: Forgetting that 1900 was not a leap year.
Instead: Century years are leap years only if divisible by 400.
Checking the method
Every golden date is rechecked with Python’s calendar functions and Zeller’s congruence.
An independent script uses Python’s ISO calendar for week numbers and Zeller’s congruence, a different formula, for weekdays.
Read how we test our calculators, or browse more everyday tools.