Day of Week Calculator
Find the day of the week, week number, and day of year for any date.
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How to use this calculator
- 1
Select any date using the date picker.
- 2
The day of the week, ISO week number, and day-of-year appear instantly.
Frequently asked questions
How is the week number calculated?
This calculator uses ISO 8601 week numbering, where week 1 is the week containing the first Thursday of the year. Weeks run Monday to Sunday.
What is "day of year"?
Day of year (also called ordinal date) is the sequential count of days from January 1. January 1 is day 1; December 31 is day 365 (or 366 in a leap year).
Can I look up any historical date?
Yes — the calculator works for any date your browser supports, typically from the year 100 through the far future.
Finding the day of the week for any date
ISO week numbering
The ISO 8601 standard defines weeks as starting on Monday and ending on Sunday. Week 1 is the week that contains the year's first Thursday. This means week 1 can start in late December of the previous year, and the last days of December can belong to week 1 of the following year.
Historical day of week
Determining the day of the week for a historical date is surprisingly complex — it requires accounting for the Gregorian calendar reform (1582 in many countries, later in others) and varying leap year rules. This calculator uses the proleptic Gregorian calendar for consistency across all dates.
Learn more from an authoritative source:
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Results are estimates for informational purposes only and do not constitute professional financial, medical, legal, or technical advice. Read full disclaimer →