complete.tools

Day of Week Calculator

Instantly find out what day of the week any date falls on, past or future.

What this tool does

Instantly find out what day of the week any date falls on, past or future. You fill in these fields: Date. It shows day of the week, full date, day of the year, days left in the year, and ISO week number.

Inputs

  • Date: A date. Starts as today's date. Defaults to today. Pick any other date to look it up.

How to use

  1. Pick a date for "Date".
  2. Read the result: day of the week, full date, day of the year, days left in the year, and ISO week number. It appears once every required field has a valid value.

Worked examples

Example 1. Inputs:

  • Date: February 10, 2023

Results:

  • Day of the week: Friday
  • Full date: Friday, February 10, 2023
  • Day of the year: 41
  • Days left in the year: 324 days
  • ISO week number: 6

How this is worked out: Jan 1 2023 is a Sunday. Feb 10 is day 31 + 10 = 41, so 40 days later; 40 mod 7 = 5, Sunday + 5 = Friday. 2023 has 365 days, 365 - 41 = 324. ISO week 1 starts Monday Jan 2; Feb 10 is 39 days after it, 1 + floor(39 / 7) = 6. February 10, 2023 was a Friday (1 Jan 2023 was a Sunday; day 41 = 31 + 10, and (41 - 1) mod 7 = 5 days after Sunday is Friday). Day of year = 31 + 10 = 41, and 365 - 41 = 324 remain. February is in quarter 1. ISO week 1 of 2023 began Monday 2 January (day 2); day 41 is 39 days later, floor(39 / 7) + 1 = week 6, and Friday is ISO weekday 5. ISO 8601: week 1 contains the first Thursday; 2023-01-02 (Monday) starts week 1. Feb 10 is day 31+10 = 41 of the year, day 41 - 2 = 39 days after Jan 2, floor(39/7) = 5, so week 6.

Example 2. Inputs:

  • Date: January 1, 2000

Results:

  • Day of the week: Saturday
  • Full date: Saturday, January 1, 2000
  • Day of the year: 1
  • Days left in the year: 365 days
  • ISO week number: 52
  • Note: This date falls on a weekend.
  • Note: This date belongs to ISO week year 1999, not calendar year 2000.

How this is worked out: Jan 1 2000 was a Saturday (Gregorian calendar). It is day 1 of a leap year of 366 days, so 365 remain. Jan 1 2000 is a Saturday, before the first Thursday (Jan 6), so it belongs to the last ISO week of 1999, week 52.

Example 3. Inputs:

  • Date: January 1, 1900

Results:

  • Day of the week: Monday
  • Full date: Monday, January 1, 1900
  • Day of the year: 1
  • Days left in the year: 364 days
  • ISO week number: 1

How this is worked out: Julian Day Number of 1900-01-01 is 2415021; (JDN + 1) mod 7 = 2415022 mod 7 = 1, and 1 means Monday with 0 = Sunday. 1900 is not a leap year (century not divisible by 400), so 365 days, 364 remain. A Monday Jan 1 starts ISO week 1. From Jan 1 1900 to Jan 1 2000 is 100 years with 24 leap days (1904 to 1996; 1900 is not a leap year): 36500 + 24 = 36524 days; 36524 mod 7 = 5. Jan 1 2000 was a Saturday, and Saturday minus 5 days is Monday.

Example 4. Inputs:

  • Date: December 31, 2024

Results:

  • Day of the week: Tuesday
  • Full date: Tuesday, December 31, 2024
  • Day of the year: 366
  • Days left in the year: 0 days
  • ISO week number: 1
  • Note: This date belongs to ISO week year 2025, not calendar year 2024.

How this is worked out: 2024 is a leap year with 366 days, so Dec 31 is day 366 and 0 remain. Jan 1 2024 is a Monday; 365 days later, 365 mod 7 = 1, Monday + 1 = Tuesday. Its ISO week runs Monday Dec 30 2024 to Sunday Jan 5 2025 and holds Thursday Jan 2 2025, the first Thursday of 2025, so it is ISO week 1 (of 2025). Last day of a leap year is day 366, none remain. December is in quarter 4. 1 Jan 2024 was a Monday and 365 mod 7 = 1, so it is a Tuesday. Its week (Monday 30 December to Sunday 5 January) holds Thursday 2 January 2025, the first Thursday of 2025, so by ISO 8601 it is week 1 of 2025.

Where this comes from

Facts about a date. The weekday, the day of the year and the ISO 8601 week are worked out from the calendar date alone, on the Gregorian calendar. The weekday comes from counting days from a known date, the day of the year from the month lengths and the leap year rule, and the week from the rule that ISO weeks start on Monday and week 1 holds the first Thursday of the year. Source: The Gregorian calendar (month lengths and the leap year rule) and ISO 8601 (the week date: weeks start on Monday, week 1 contains the first Thursday).

Limitations

  • Works on calendar dates only, with no time of day or time zone.
  • Uses the Gregorian calendar for every date, including dates before it was adopted in 1582 or later by some countries.
  • The starting date comes from your device's date when the page opens.

FAQs

Q: What date does it start with?
A: Today's date on your device when the page opens. Pick any other date to look that one up.
Q: Which dates does it accept?
A: Any real calendar date from the year 0001 to 9999. A date that does not exist, such as February 29 in a year that is not a leap year, is refused.
Q: What is the ISO week number?
A: It is the week of the year under ISO 8601, where weeks start on Monday and week 1 is the week with the first Thursday of the year. The first days of January can belong to the last week of the previous year, and the last days of December to week 1 of the next year.
Q: Are leap years handled?
A: Yes. The day of the year, the days left in the year and the length of the year follow the Gregorian leap year rule: every fourth year, except century years not divisible by 400.

Explore Similar Tools

Explore more tools like this one: