Day of the Week Calculator
Find the weekday of any date — past or future — with its day of the year, days remaining in the year, ISO week number and whether the year is a leap year.
The calculator uses the Gregorian calendar (valid for dates from 15 October 1582).
How the day of the week calculator works
The calculator uses the Gregorian calendar (valid for dates from 15 October 1582). The ISO week number treats Monday as the first day and week 1 as the week containing the first Thursday of the year, so a date in early January can belong to week 52 or 53 of the previous year.
Formula: ISO week = week containing the year's first Thursday = week 1
Worked examples
| Inputs | Day of the week | Note |
|---|---|---|
| 1 January 2000 | Saturday | Saturday |
| 31 December 2024 | Tuesday | Tuesday, ISO week 1 of 2025 |
| 20 July 1969 | Sunday | Sunday |
FAQFrequently asked questions
What day of the week was I born on?
Enter your birth date; the result is the weekday.
Why is 31 December sometimes in week 1?
ISO weeks start on Monday and week 1 holds the first Thursday of the year, so the last days of December can belong to the next year's week 1.
How many weeks does a year have?
52 ISO weeks usually; years that start on a Thursday (or a leap year starting on Wednesday) have 53.
Is 2100 a leap year?
No — century years are leap years only when divisible by 400. 2000 was, 2100 is not.
Does this work for dates before 1582?
It applies the Gregorian rules backwards (proleptic); historical dates before 1582 were in the Julian calendar.
Where these figures come from
- ISO 8601 — Date and time — Representations for information interchange — the yyyy-mm-dd format and the week-numbering rules used here
- US Naval Observatory — Calendars — the Gregorian leap-year rule
- GOV.UK — bank holidays — bank holidays by nation (not applied here)
Last checked: September 2026. The Gregorian calendar rules do not change; working-day counts exclude Saturdays and Sundays only.