Calendar desk
Day of the Week
Identify the weekday for a calendar date.
Day of the Week: Look up which weekday belongs to a particular calendar date, such as a birthday, booking or historical anniversary. The result gives the weekday name and a number from Monday = 1 to Sunday = 7. It answers a date question without converting an instant between time zones. Runs 100% locally in your browser with zero server file uploads.
- Category
- Calculators
- Runs
- In your browser
- Cost
- Free · no sign-up
- Availability
- Ready to use
Day of week
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- ISO weekday number
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Weekday = UTC calendar weekday for the date- Dates are calendar dates at UTC midnight; no local timezone or daylight-saving adjustment is applied.
- Results are planning utilities. Public holidays, locale-specific work rules, and company policies are not inferred.
- Monday is ISO weekday 1 and Sunday is ISO weekday 7.
Weekday formula and leap-day working
One way to express the weekday rule is n = 1 + ((A − 1) mod 7), where A is the Gregorian ordinal date with 1 January of year 1 assigned ordinal 1, and mod gives a remainder from 0 to 6. The seven values correspond in order to Monday, Tuesday, Wednesday, Thursday, Friday, Saturday and Sunday.
For 2024-02-29, A = 738945. Subtracting one gives 738944 = 105563 × 7 + 3, so n = 1 + 3 = 4. The widget therefore shows Thursday and weekday number 4. This is a lookup of one date, with no inclusive or exclusive interval to choose.
Reading weekday numbers correctly
Python's public date documentation (https://docs.python.org/3/library/datetime.html#datetime.date.isoweekday) describes the same Monday = 1 convention. Numbering systems that begin at zero or put Sunday first require a conversion before you compare their numbers with this page.
A weekday says nothing by itself about whether a workplace is open. Thursday may be a public holiday, and a weekend operation may work on Sunday. Use the name to place an appointment on a calendar, then apply the opening hours relevant to that appointment. For dates before historical calendar reforms, this lookup continues Gregorian rules rather than reproducing local Julian calendars.
How to use it
- Enter the year, month and day you want to identify.
- Calculate to obtain the weekday name.
- Read the weekday number using the Monday-to-Sunday scale.
Privacy & limitations
Dates, times and results stay in your browser. No inputs are uploaded.
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Frequently asked questions
Why does Sunday have the number seven?
This page uses Monday = 1, Tuesday = 2 and so on through Sunday = 7. A calendar that visually starts its rows on Sunday does not change the numbering used by this result.
Can a different time zone change the weekday shown?
The entered date is taken literally. There is no time or location field. An actual instant may fall on different dates in different places, so establish the relevant local date before looking up its weekday.
Does the lookup accept 29 February in every year?
No. A date must exist in the Gregorian calendar. A leap-day date is accepted in a leap year, while an impossible date produces an error instead of being moved into March.
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