Day of the Week Calculator
Find the weekday, day of year and ISO week for any Gregorian date.
Result
Day of week: Sunday
ISO weeks start Monday. Week 1 is the week containing the year’s first Thursday, so its week-year can differ near New Year.
- Day of year
- 263
- ISO week date
- 2026-W38-7
- Calendar year
- Common year
https://calcgallery.com/date-time/day-of-week-calculator/
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How to use the Day of the Week Calculator
- Enter any Gregorian calendar date.
- Read its weekday and ordinal day of the year.
- Use the ISO week date when a Monday-based week number is required.
Weekday, ordinal day and ISO week
These are three views of one date. The weekday repeats every seven days, the ordinal day counts from January 1, and the ISO week date groups days into Monday-to-Sunday weeks.
The ISO week-year is shown explicitly because using only a week number near January 1 or December 31 can identify the wrong year.
With your numbers
- 2026-09-20 falls on Sunday
- It is day 263 of the year and ISO week date 2026-W38-7
Quick reference
ISO weeks start Monday. Week 1 is the week containing the year’s first Thursday, so its week-year can differ near New Year.
| Example | Day of week |
|---|---|
| September 20, 2026 | Sunday |
| Leap day 2024 | Thursday |
| ISO week-year boundary | Friday |
| Unix epoch date | Thursday |
| New Year 2026 | Thursday |
| Gregorian century exception | Thursday |
Common questions about the Day of the Week Calculator
- Why can January belong to the previous ISO week-year?
- ISO week 1 is the week containing the year’s first Thursday. Dates near New Year can therefore belong to an adjacent week-year.
- Do ISO weeks start on Sunday?
- No. ISO 8601 weeks begin on Monday and number weekdays from Monday 1 through Sunday 7.
- What is an ordinal day?
- It is the date’s position in the year: January 1 is day 1 and December 31 is day 365 or 366.