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Day of the Week Calculator

Find the weekday of any date from the year 1 to 9999, plus the day of the year, days left in the year, quarter, leap year status and ISO week number.

Any date from the year 1 to 9999, using the Gregorian calendar throughout.

How to use the calculator

Pick a date and press Find the day. The date box starts on today, so you can also just press the button to learn about the current date. The result shows the weekday, and below it you find:

  • Day of the year, for example day 289 of 365
  • Days left in the year after the chosen date
  • Quarter: Q1 is January to March, Q2 April to June, Q3 July to September and Q4 October to December
  • Leap year: whether the year has a 29 February
  • ISO week: the week number used in many business and government calendars, written like 2026-W42-5

How the weekday is found

The calculator counts the days from a fixed reference date to your date and takes the remainder after dividing by 7. A few facts make the pattern easy to follow:

  • A common year has 365 days, which is 52 weeks and 1 day. So a date usually moves forward by one weekday each year, and by two when 29 February falls in between. October 16 was a Friday in 2026, is a Saturday in 2027 and a Monday in 2028.
  • Within a century that has a leap year every four years, such as 1901 to 2099, the whole calendar repeats every 28 years. For example, 1998, 2026 and 2054 all begin on a Thursday.
  • A year divisible by 100 but not by 400, such as 2100, is not a leap year and breaks the 28-year pattern.
  • The full Gregorian cycle is 400 years, which is 146,097 days or exactly 20,871 weeks. After 400 years every date falls on the same weekday again.
  • In any year, 4 April, 6 June, 8 August, 10 October and 12 December share the same weekday. In 2026 they are all Saturdays.

Worked example

For 16 October 2026: January to September have 273 days, so 16 October is day 289. 2026 is not a leap year, so 365 − 289 = 76 days remain. The date is a Friday in Q4, and it lies in ISO week 42.

How ISO week numbers work

ISO 8601 weeks start on Monday. Week 1 is the week that contains the year's first Thursday. Because of this, a few days at the start of January can belong to the last week of the previous year, and late December days can belong to week 1 of the next year. For example, 3 January 2021 is in week 53 of 2020, and 30 December 2024 is in week 1 of 2025. The number after the week (1 for Monday to 7 for Sunday) is the ISO day of the week.

Good to know

The calculator uses the proleptic Gregorian calendar: today's leap year rules are applied to every year back to the year 1. Many countries used the Julian calendar before 1582 or later, so the weekday for an old historical date may differ from what records of that time show.

Frequently asked questions

Which years can I enter?

Any date from 1 January of the year 1 to 31 December 9999. All dates use the Gregorian calendar rules, even those before the calendar was introduced in 1582.

Why does 1 January sometimes belong to week 52 or 53?

Under ISO 8601, week 1 is the week that contains the first Thursday of the year, and weeks start on Monday. If 1 January falls on a Friday, Saturday or Sunday, that day still belongs to the last week of the previous year.

How do I know if a year is a leap year?

A year is a leap year if it is divisible by 4, except years divisible by 100, which are leap years only if they are also divisible by 400. So 2024 and 2000 are leap years, while 1900 and 2100 are not.

Can I reuse an old calendar for a future year?

Yes, when both years start on the same weekday and are both leap years or both common years. Between 1901 and 2099 this happens every 28 years, so a 1998 calendar fits 2026 and 2054.