Day of the Week Calculator
Find the day of the week for any date, plus its day of the year and ISO week number.
How it works
day of year = date − January 1 + 1 · week number = ISO-8601 (Monday-start weeks)
The weekday comes straight from the calendar, using the same proleptic Gregorian rules computers and modern records use. Day of the year counts January 1 as day 1, so December 31 is day 365 — or 366 in a leap year. The week number follows the ISO-8601 standard: weeks run Monday to Sunday, and week 1 is the week containing the year’s first Thursday.
Worked example
September 11, 2026 is a Friday. It is day 254 of 365 (January through August use up 243 days, plus 11), with 111 days left, and it sits in ISO week 37 of 2026.
Frequently asked questions
Why does my planner show a different week number?
Many US calendars start weeks on Sunday and call the week containing January 1 “week 1”. The ISO standard used here starts weeks on Monday and numbers by the first Thursday, so the two systems can differ by one.
Do historical dates match the weekday shown?
Only back to the Gregorian switch (1582 in Catholic Europe, 1752 in Britain and its colonies). Earlier dates were recorded in the Julian calendar, so a history book’s weekday can differ from the proleptic result here.
How can I work out weekdays in my head?
The Doomsday rule: in any year, 4/4, 6/6, 8/8, 10/10, 12/12 and the last day of February all share a weekday. Learn that anchor day for the year and step to your date from the nearest one.
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This calculator is for educational purposes. Double-check important results before acting on them.