ToolFoxDay of the Week Finder
Find what day of the week any date falls on, plus its day-of-year, ISO week number and leap-year status.
Pick a date to see its weekday and calendar facts.
How it works
The weekday comes directly from JavaScript's calendar-aware Date object, constructed at local midnight to avoid timezone drift. Day-of-year counts the number of days elapsed since 1 January of the same year. The ISO week number follows the ISO-8601 standard: weeks start on Monday, and week 1 is defined as the week containing the year's first Thursday — which is why a date can belong to an ISO week number from the previous or next calendar year, as with 1 January 2027 falling in week 53 of 2026.
Example
15 May 1990 was a Tuesday, the 135th day of that year. 1 January 2000 was a Saturday, and 2000 was a leap year because it is divisible by 400 — an exception to the usual "divisible by 100 is not a leap year" rule.
Frequently asked questions
Why does 1 January sometimes fall in the previous year's ISO week?
ISO-8601 week numbering is based on which week contains the year's first Thursday, not the calendar year of 1 January itself — so early-January dates can belong to week 52 or 53 of the previous year, as happens for 2027.
How is a leap year determined?
A year is a leap year if divisible by 4, except century years (divisible by 100) unless also divisible by 400 — which is why 2000 was a leap year but 1900 was not.
Does the day-of-year count include the date itself?
Yes — 1 January is day 1, so day-of-year is a 1-indexed count of elapsed calendar days including the selected date.
Is my date sent anywhere?
No — everything is computed locally in your browser using standard date arithmetic.
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