Plain DatesClear date calculations, with the rules shown.

Weekday lookup

What day of the week was a date?

Look up the weekday for a calendar date and see its position in the year and ISO week calendar.

Choose a date

Three ways to locate a date

The weekday repeats on a seven-day cycle. The day-of-year number counts from January 1, while the ISO week value groups dates into Monday-through-Sunday weeks. Each answers a different scheduling question.

New Year example

December 30, 2024 is a Monday and belongs to ISO week 1 of 2025. The ISO week-year is shown explicitly so it is not mistaken for the calendar year.

Historical limitation

The page applies modern Gregorian rules consistently. It does not model country-specific historical calendar adoption before 1900.

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