Day of the Week Calculator

Find out what day of the week any date in history or the future falls on

About Day Calculations

Our calculator uses the Zeller's Congruence algorithm, a well-known method for calculating the day of the week for any Julian or Gregorian calendar date. The formula accounts for:

  • Different month lengths (28-31 days)
  • Leap years (every 4 years, except century years not divisible by 400)
  • The Gregorian calendar reform in 1582

Dates before October 1582 use the Julian calendar, while later dates use the Gregorian calendar (with the 10-day adjustment for countries that adopted it later).

How It Works

The calculation follows these steps:

  1. Adjust month and year if January or February
  2. Calculate century and year within century
  3. Apply Zeller's Congruence formula:

    h = (q + ⌊(13(m+1)/5⌋ + K + ⌊K/4⌋ + ⌊J/4⌋ + 5J) mod 7
    Where h is day code (0=Sat, 1=Sun, 2=Mon,...,6=Fri)

  4. Convert result to weekday name

Famous Dates

  • January 1, 2000: Saturday (Y2K)
  • July 4, 1776: Thursday (US Independence)
  • November 22, 1963: Friday (JFK Assassination)
  • December 25, 2023: Monday (Christmas)
  • February 29, 2024: Thursday (Leap Day)

Did You Know?

The Gregorian calendar repeats every 400 years (20871 weeks exactly). This means January 1, 2023 and January 1, 2423 will fall on the same day (Sunday).

Day of the Week FAQ

Our calculator is highly accurate for all dates from 1582 onward (Gregorian calendar). For dates before October 1582, it uses the Julian calendar system which was in effect at that time.

Note about calendar reform: When the Gregorian calendar was introduced in 1582, 10 days were skipped to correct drift in the Julian calendar. Different countries adopted the Gregorian calendar at different times:

  • October 1582: Catholic countries (Italy, Spain, Portugal, France)
  • September 1752: Great Britain and colonies (including America)
  • 1918: Russia (after the revolution)

Our calculator shows the astronomical date, not necessarily the date that would have been recorded in a particular country at the time.

The calculation uses Zeller's Congruence, a well-known algorithm developed by Christian Zeller in 1887. The formula is:

h = (q + ⌊(13(m+1)/5⌋ + K + ⌊K/4⌋ + ⌊J/4⌋ + 5J) mod 7

Where:

  • h is day code (0=Saturday, 1=Sunday, 2=Monday,...,6=Friday)
  • q is day of the month
  • m is month (3=March, 4=April,...,14=February)
  • K is year of century (year mod 100)
  • J is zero-based century (⌊year/100⌋)

For January and February, we consider them as months 13 and 14 of the previous year. This formula works for both Julian and Gregorian calendars.

Knowing the day of the week for any date has many practical applications:

  • Event Planning: Check what day a future date falls on for scheduling weddings, conferences, etc.
  • Historical Research: Understand the weekday of important historical events
  • Genealogy: Verify dates in family history records
  • Business: Calculate delivery dates, payment due dates, or project timelines
  • Education: Teach calendar concepts and date calculations
  • Personal: Find out what day you were born or other significant personal dates
  • Astronomy: Correlate dates with astronomical events

For example, knowing that July 20, 1969 (Moon landing) was a Sunday helps understand media coverage patterns of the event.