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Time Utility

Day of the Week Calculator

Find which weekday any calendar date falls on, alongside week numbers and leap year info.

Enter Calendar Date

Enter valid date parameters.
Time Mechanics

The Gregorian
Calendar & Weekdays

Our modern calendar, the Gregorian calendar, is solar-based and consists of 365 days in a standard year, and 366 days in a leap year. Determining the day of the week relies on cycles of exactly 7 days.

Cycle of 7

The week is a repeating cycle of exactly seven days that has run uninterrupted for centuries, independent of astronomical cycles.

Julian Reform

Introduced by Julius Caesar in 46 BC to add leap years every four years. Later adjusted by Pope Gregory XIII in 1582.

Century Rules

Centennial years are not leap years unless divisible by 400. This maintains calendar alignment with solar seasons.

How Do Leap Years Work?

To keep our calendar year aligned with the astronomical solar year (which is approximately 365.2422 days), an extra day (February 29) is added to the calendar every four years. The rules for leap years under the Gregorian calendar are:

  • The year must be evenly divisible by 4.
  • If the year is evenly divisible by 100, it is NOT a leap year, unless...
  • The year is also evenly divisible by 400. E.g., 2000 was a leap year, but 1900 and 2100 are not.

Calendar Trivia:

  • • Doomsday Algorithm: A method designed by mathematician John Conway to calculate the weekday of any date in your head.
  • • Missing Days: To correct the solar offset, October 5-14, 1582 never existed in Catholic European countries.
  • • ISO Week Numbers: ISO 8601 specifies that the first week of a year is the week containing the first Thursday.

How the calculation works

A known anchor date fixes the weekday. 1 January 2024 was a Monday.

Weekday = anchor + (day difference mod 7)

The remainder 0 is Monday if the anchor is Monday.

Worked example: 11 October 2026

  1. 1 January 2024 to 11 October 2026 is 1,014 days.
  2. 1,014 mod 7 = 6.
  3. Monday plus 6 days is Sunday.

Weekday shifts from a Monday anchor

Days later mod 7Weekday
0Monday
1Tuesday
6Sunday

Frequently asked questions

What day of the week is 11 October 2026?▼
Sunday.
What day was 1 January 2024?▼
Monday.
Why use modulo 7?▼
Weekdays repeat every 7 days.