Day of the Week
Find out what day of the week any past or future date falls on, with day-of-year numbers and historical facts.
Weekend day
#201
out of 365/3661969
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What Is a Day of the Week?
A day of the week calculator determines the specific weekday (Sunday through Saturday) for any date in past or future history. It uses modular calendar mathematics (such as Zeller’s Congruence or the Doomsday rule) to instantly find the weekday for any event.
How to Use This Calculator
- Select any calendar date from the date picker or choose a famous historical preset.
- Instantly view the day of the week (e.g., Sunday, Monday).
- Review the day-of-the-year number (e.g. Day #201 of 365) and weekday status.
Zeller’s Congruence Algorithm
h = \left( q + \left\lfloor \frac{13(m+1)}{5} \right\rfloor + K + \left\lfloor \frac{K}{4} \right\rfloor + \left\lfloor \frac{J}{4} \right\rfloor - 2J \right) \pmod{7}Where q is day of month, m is shifted month, K is year of century, and J is zero-based century.
Worked Example
Scenario: What day of the week was the Apollo 11 Moon landing on July 20, 1969?
Date: July 20, 1969.
Apply calendar modulo math.
July 20, 1969 was Day #201 of the year 1969.
Tips & Key Notes
- The United States Declaration of Independence was signed on July 4, 1776, which was a Thursday.
- There are 7 possible calendar year layouts (starting on each day of the week) plus 7 leap year variations, giving 14 total possible yearly calendars.
- The Gregorian calendar repeats its exact day-of-the-week sequence every 400 years (146,097 days, which is exactly divisible by 7).
Frequently Asked Questions
What was the day of the week for January 1, 2000?
January 1, 2000 was a Saturday.
How does the calculator determine weekdays for old dates?
It calculates using the standard Gregorian calendar algorithm, factoring in leap years and century rules.
What is the Doomsday rule?
The Doomsday rule is a mental math algorithm created by mathematician John Conway that allows someone to quickly calculate the day of the week for any date.
Does the calendar cycle ever repeat exactly?
Yes, the calendar repeats in cycles of 6, 11, 11, and 28 years, and repeats identically every 400 years.
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