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AgeWise Calculator

Day of the Week Calculator

Enter any date to see which weekday it falls on, its day number within the year, its ISO week and whether that year is a leap year.

What is a day of the week calculator?

A day of the week calculator tells you whether a given date is a Monday, a Tuesday or any other day. That sounds simple for next week, but it quickly becomes awkward for a date decades away, or one long in the past, because month lengths vary and leap years add an extra day roughly every four years. Doing it by hand means counting forward through every one of those irregularities.

Alongside the weekday, this tool reports three related facts that people often need at the same time: the date’s position in the year (day 1 to day 365 or 366), its ISO 8601 week number, and whether its year is a leap year. All four are worked out in your browser from the Gregorian calendar rules.

How to use it

  1. Choose a date with the date picker, or type it in.
  2. Press “Find the Day”.
  3. Read the weekday in the heading, then the day of the year, ISO week and leap-year status in the cards below it.

Typical uses include checking which day a birthday or anniversary falls on in a future year, confirming the weekday printed on an old document, planning an event that must land on a weekend, and finding the week number a colleague mentioned in a schedule.

Worked examples

Example 1: a historical date

4 July 1776 was a Thursday. It was day 186 of 1776, which was a leap year, so 29 February had already been counted. It sits in ISO week 27, the week running from Monday 1 July to Sunday 7 July 1776 (counted in the modern calendar).

Example 2: a New Year's Day in last year's week

1 January 2000 was a Saturday and day 1 of a leap year. Its ISO week, though, is week 52 of 1999, because the first Thursday of 2000 was 6 January. That week ran from Monday 27 December 1999 to Sunday 2 January 2000.

Why the weekday shifts each year

A common year has 365 days, which is 52 weeks plus one day. So a date usually moves one weekday later each year: 11 October was a Saturday in 2025 and is a Sunday in 2026. When a 29 February falls between the two dates, the shift is two days instead. That is why birthdays do not simply cycle through the week in order, and why a quick lookup is easier than counting.

Frequently asked questions

Which calendar does the calculator use?

It uses the Gregorian calendar for every date, including dates before a country adopted it. Historical records written in the older Julian calendar can therefore show a different weekday.

How is the day of the year counted?

1 January is day 1. In a common year 31 December is day 365; in a leap year it is day 366, because 29 February is added as day 60.

Why can the ISO week belong to a different year?

ISO weeks run Monday to Sunday and week 1 is the week containing the year's first Thursday. Days at the very start or end of January and December can therefore belong to a week of the neighbouring year.

What makes a year a leap year?

A year divisible by 4 is a leap year, except century years, which must also be divisible by 400. So 2000 was a leap year but 1900 and 2100 are not.

Related tools

For the full Monday-to-Sunday range of a week, use the Week Number calculator.