Day of the Week Calculator
Free day of the week calculator. Find the weekday for any past or future date, plus its day-of-year number and ISO 8601 week number.
Day of the Week Calculator
Background.
This day of the week calculator finds the weekday for any date, past or future, along with its position in the calendar year (day-of-year) and its ISO 8601 week number. Enter any date and the calculator returns the weekday name immediately, computed directly from the proleptic Gregorian calendar rather than a lookup table — so it works identically for a date next month or a date from a century ago.
The ISO week number is the trickiest of the four outputs, because the ISO 8601 standard defines week 1 as 'the week containing the year's first Thursday' rather than simply 'the week containing January 1st'. This means a date in very early January can actually belong to the last week (52 or 53) of the PREVIOUS ISO year, and a date in very late December can belong to week 1 of the FOLLOWING year — the calculator handles both directions correctly using the standard's own canonical algorithm.
What is day of the week calculator?
The day of the week is one of the seven repeating weekday labels (Sunday through Saturday) associated with any calendar date under the Gregorian calendar. ISO 8601 additionally assigns each weekday a number from 1 (Monday) to 7 (Sunday) — deliberately starting the week on Monday rather than Sunday, which is the international standard used in most of Europe, and in business and engineering contexts worldwide, even though many calendars and computer systems default to Sunday-first display. The ISO week number is a separate concept: a running count of 7-day weeks within a year, defined so that week 1 always contains the year's first Thursday, which guarantees every ISO week has a majority of its days in the year it's numbered for.
How to use this calculator.
- Enter any date in ISO 8601 format (YYYY-MM-DD).
- Read the primary 'Day of the week' result immediately.
- Check 'Weekend?' for a quick yes/no on whether the date falls on a Saturday or Sunday.
- Use 'Day of the year' to find the date's ordinal position (useful for Julian-date-style references in some industries).
- Use 'ISO week number' for project planning, payroll periods, or any system that tracks time by week number rather than by calendar date.
The formula.
The weekday is read directly from the UTC day-of-week of the entered date. The ISO weekday number remaps that (where 0=Sunday) to the ISO 8601 convention (1=Monday through 7=Sunday) by treating Sunday as 7 instead of 0. Day-of-year is the count of days from January 1st of the same year up to and including the entered date. The ISO week number follows the ISO 8601 standard's canonical method: shift the date to the Thursday of its own week (Monday-anchored), find the year that Thursday falls in (the 'ISO year', which can differ from the calendar year for dates in very early January or very late December), locate that ISO year's own first Thursday the same way, and count the number of whole weeks between the two Thursdays, plus one.
A worked example.
July 27, 2026 falls on a Monday — ISO weekday number 1. It is the 208th day of 2026 (31 January + 28 February + 31 March + 30 April + 31 May + 30 June + 27 July = 208). Its ISO week number is 31, since January 1, 2026 is itself a Thursday and therefore anchors ISO week 1 cleanly at the start of the year, making every subsequent Monday-to-Sunday span an easy running count up to week 31 by late July. Contrast this with January 1, 2021, which was a Friday: because ISO week 1 must contain the year's first Thursday, and the nearest Thursday to January 1, 2021 is December 31, 2020, that date actually belongs to ISO week 53 of 2020 — not week 1 of 2021 — which is the canonical textbook example of how the ISO week-numbering system can assign a date to the adjacent year's week count.
Frequently asked questions.
Why does ISO week numbering sometimes put January 1st in the previous year's week count?
Why does the ISO week start on Monday instead of Sunday?
Can I use this to find what day of the week I was born on?
Does this work for dates before the Gregorian calendar was adopted?
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