Day of Week from Date (Past, Present, Future)

Quick lookup: what day of the week falls on any date? Useful for scheduling, historical reference, birthday curiosity ("I was born on a Wednesday"). Supports dates 1900-2100; uses the Gregorian calendar (rejected for dates pre-1582 / 1700 / 1752 depending on country).

How to use the Day of Week from Date (Past, Present, Future)

Pick a date. The output shows the day of the week, ISO 8601 week number, day of year, and a calendar view of the month containing the date.

Finding the day of the week for any date

Every date falls on a fixed day of the week, derivable by formula (Zeller's congruence is the classic one) or, as here, by the calendar engine built into your browser. Alongside the weekday, this tool shows the ISO 8601 week number and the day-of-year — the values schedulers and spreadsheets often actually need.

It uses the proleptic Gregorian calendar across 1900–2100. For very old dates that matters: countries switched from the Julian calendar at different times (1582 in much of Catholic Europe, 1752 in Britain and its colonies), so a "historical" weekday before the switch depends on which calendar the source used.

Common use cases

  • Scheduling — check which weekday a future date lands on before you book it.
  • "What day was I born?" — the classic personal lookup.
  • ISO week numbers — get the week-of-year used in fiscal calendars and many APIs.
  • Planning recurring events — see at a glance whether a date is a weekday or a weekend.
  • Historical reference — find the weekday of a past event within the Gregorian range.

Frequently asked questions

What is an ISO 8601 week number?
ISO weeks start on Monday, and week 1 is the week containing the year's first Thursday. That means a date in early January can belong to week 52 or 53 of the previous year, which trips up a lot of spreadsheets.

Why is the range limited to 1900-2100?
It is a practical window that keeps results unambiguous and avoids the Julian-to-Gregorian switchover, where the "correct" weekday depends on the country and source.

Does the week start on Sunday or Monday?
The ISO week number assumes a Monday start. Note that the US convention often treats Sunday as the first day of the week, so a "week number" elsewhere may differ by one.

How is the weekday actually computed?
From the browser's Gregorian calendar engine, which is equivalent to applying Zeller's congruence — a closed-form formula that maps any date to its day of the week.
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