How is the day of the week determined?

Using JavaScript's Date object, which follows the proleptic Gregorian calendar (projecting the current calendar back in time). For dates before the 1582 Gregorian reform, historical Julian calendars may differ.

What is ISO week number?

ISO 8601 week numbering: weeks start Monday, week 1 contains the first Thursday of the year. Ranges 1-52 or 1-53.

Day of the Week Calculator

Date & Time· 11 tools

Free day-of-week calculator. Enter any date from year 1 to 9999 and see which day of the week it was or will be — Monday, Tuesday, etc. Also shows ISO week number and day-of-year.

Date

That date is a

Monday

Monday, June 22, 2026

ISO Week

Day of Year

173

Leap Year

Frequently Asked Questions

References

ISO 8601 — Date and time representations — International Organization for Standardization

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Frequently Asked Questions

References and sources

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