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Day of week calculator
Enter any date — its weekday and ISO week number appear instantly, valid for any year the Gregorian calendar covers.
Calculator
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Day of week
ISO week—
How it works
Every date maps to a fixed position in the repeating 400-year Gregorian weekday cycle. The calculator resolves your date onto that cycle directly — no lookup tables, valid for any year from 1583 (the Gregorian adoption) onward.
Frequently asked questions
How far back is this valid?
For dates after the Gregorian calendar's adoption in 1582. Earlier dates follow the Julian calendar, whose weekdays drift from this result.
What is the ISO week number?
ISO-8601 weeks start on Monday; week 1 is the week containing the first Thursday of the year. Year boundaries can therefore belong to week 52 or 53 of the previous year.
Why does the weekday of a fixed date never change?
The Gregorian calendar repeats exactly every 400 years — 146,097 days, a whole multiple of 7 — so a given month and day always lands on the same weekday within its cycle position.