Day of the Week Calculator

Find out which day of the week a specific date falls on. This calculator can determine the day of the week for any date in the past or future.

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How It Works

The day of the week calculator uses algorithms to determine which day of the week (Monday through Sunday) corresponds to any given date. This is useful for planning events, understanding historical dates, or simply satisfying curiosity about which day of the week you were born on or when a future event will occur.

Calendar Systems and Weekdays

The seven-day week originated in ancient Mesopotamia and was later adopted by many cultures around the world. The modern Gregorian calendar, used internationally today, maintains this seven-day cycle without interruption across months and years. Each day of the week has historical and cultural significance:

  • Monday - Named after the Moon (Moon's day). In many cultures, it's considered the first day of the work week.
  • Tuesday - Named after Tiu or Tyr, the Norse god of war (similar to Mars in Roman mythology).
  • Wednesday - Named after Woden or Odin, the chief Norse deity (similar to Mercury).
  • Thursday - Named after Thor, the Norse god of thunder (similar to Jupiter).
  • Friday - Named after Frigg or Freya, the Norse goddess of love and beauty (similar to Venus).
  • Saturday - Named after Saturn, the Roman god of agriculture.
  • Sunday - Named after the Sun (Sun's day). Traditionally considered the first day of the week in many religious contexts.

Day of Week Algorithms

Several mathematical algorithms can determine the day of the week for any date. Some of the most well-known include:

  • Zeller's Congruence - A formula developed by Christian Zeller in the 19th century that calculates the day of the week for any Gregorian calendar date.
  • Gauss's Algorithm - Developed by mathematician Carl Friedrich Gauss, this method calculates the date of Easter but can be adapted for general day-of-week calculations.
  • Sakamoto's Algorithm - A simple method using lookup tables that's efficient for computer programming.
  • Tomohiko Sakamoto's Method - A compact algorithm commonly used in computer programming due to its simplicity and efficiency.

Calendar Reforms and Day of Week Calculations

When calculating days of the week for historical dates, it's important to account for calendar reforms:

  • The transition from the Julian to the Gregorian calendar in 1582 resulted in 10 days being skipped (October 4 was followed by October 15).
  • Different countries adopted the Gregorian calendar at different times, creating historical discrepancies.
  • The start of the year varied by region and time period (e.g., March 25 in England until 1752).
  • Some historical calendars used different day-naming conventions or week structures.

Interesting Facts

  • The day of the week repeats in the same pattern every 400 years in the Gregorian calendar.
  • Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400.
  • January 1, 2000 was a Saturday.
  • Any month starting on a Sunday will always have a Friday the 13th.
  • The 13th day of the month is slightly more likely to be a Friday than any other day of the week.
  • The same calendar can be reused every 28 years, provided there's no change in the leap year pattern.