Day of the Week Calculator
Find which day of the week any date falls on, past or future.
What the Day of the Week Calculator does
The weekday of any date follows from the Gregorian calendar rules. This also reports the ISO week number, the day of the year, and which occurrence of that weekday it is within the month — useful for rules like "the third Thursday".
Formula
Weekday derived from the Gregorian calendarISO week: week 1 contains the first Thursday of the year
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Date | date | Yes | — |
How to use it
- Enter Date.
- Select Calculate.
Worked example
What day is 25 December 2026?
- Date
- 2026-12-25
Friday — day 359 of 365, in ISO week 52, the fourth Friday of December.
Reading the result
- The occurrence figure answers rules such as "the third Thursday of the month", which is how many recurring meetings and deadlines are defined.
- The ISO week number treats Monday as the first day and assigns week 1 to the week containing the first Thursday, so early January can fall in week 52 of the previous year.
Assumptions and limitations
- The Gregorian calendar is applied throughout, including for dates before it was adopted locally — countries switched in different years.
Common mistakes
- Assuming the ISO week matches a week number counted simply from 1 January.
- Expecting week 1 to always contain 1 January. It does not when the year starts late in the week.
Frequently asked questions
What is an ISO week number?
A numbering where week 1 is the week containing the first Thursday of the year, and weeks run Monday to Sunday. A year has 52 or 53 ISO weeks.