Day of the week calculator
Which day of the week a date falls on.
Fill in the fields and the result will appear here automatically.
Finds the weekday, ordinal day and ISO week with its week-year for a Gregorian calendar date. Near New Year the ISO week-year can differ from the calendar year. The weekend flag marks Saturday and Sunday without a public-holiday calendar.
How it works
Formula and logic
Gregorian leap years are divisible by 4, except century years must be divisible by 400. ISO weeks start on Monday; week 1 contains the year’s first Thursday. Each week’s Thursday determines its ISO year, so December may belong to the next ISO year and January to the previous one.
Example
29 February 2024 is Thursday, day 60, ISO week 9 of 2024. 31 December 2024 is Tuesday in ISO week 1 of 2025; 1 January 2023 is Sunday in week 52 of 2022.
Fields and units
- Date — date
How to use
- — Enter a calendar date that exists.
- — Read the weekday and the ordinal day of the year.
- — Near New Year, read the ISO week number together with its week-year; the weekend flag means Saturday or Sunday.
Method and limitations
- Calculation method
- Formula and logic
- Data or methodology source
- Python datetime: the ISO week and its separate week-year
- Limitation
- Gregorian dates without Julian conversion. Weekend means Saturday/Sunday only, without public holidays or shift schedules.
FAQ
Does the timezone matter?
No. The date is read as a plain calendar date, so the answer is the same everywhere.
Why does 1 January sometimes belong to the previous year?
Under ISO 8601 week one is the week containing the first Thursday. A year starting on a Friday, Saturday or Sunday begins in the last week of the year before.
Does it work for past centuries?
Historical dates also use the Gregorian calendar, with no Julian conversion. Countries adopted it at different times; first establish the calendar used by an old document. The form does not model local historical date omissions.
Are leap days handled?
Yes. 29 February exists only in leap years, and the day-of-year count shifts accordingly.