Perpetual Calendar

Day of the Week Calculator

Find out what day of the week any historical or future date fell on or will fall on. Powered by perpetual calendar algorithms with calendar reform notes and historical leap year facts.

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Weekday Result

September 12, 2026

Weekend / Weekday Weekend

Day of Year Day 255

ISO Week Week 37

Leap Year? No (365 days)

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The Mathematics: Zeller's Congruence & Doomsday Rule

How It Works

Calculating the day of the week for any Gregorian calendar date uses Zeller's Congruence, an algorithm devised by German mathematician Christian Zeller in 1882:

h = (q + floor(13(m+1)/5) + K + floor(K/4) + floor(J/4) - 2J) mod 7

Where q is the day of the month, m is the shifted month (March=3, January=13, February=14 of the prior year), K is the year of the century, and J is the zero-based century.

Renowned mathematician John Conway also invented the Doomsday Rule, utilizing anchor dates (e.g. 4/4, 6/6, 8/8, 10/10, 12/12) that share the same weekday every year.

The 1582 & 1752 Calendar Anomaly

Historical Context

Prior to October 1582, Europe used the Julian calendar instituted by Julius Caesar in 45 BC. Because the Julian system assumed the solar year was exactly 365.25 days (rather than 365.2422 days), it accumulated 1 day of error every 128 years.

Pope Gregory XIII reformed the calendar in 1582, dropping 10 days (October 4 was followed by October 15). The British Empire and American colonies adopted the Gregorian calendar in September 1752, skipping 11 days (September 2 was followed by September 14).

Frequently Asked Questions

Day of the Week Questions

What day of the week was the Apollo 11 moon landing?

July 20, 1969 occurred on a Sunday. Neil Armstrong and Buzz Aldrin stepped onto the lunar surface at 02:56 UTC on Monday, July 21 (Sunday evening in the United States).

Why does the day of the week shift each year?

A standard year contains 365 days (52 weeks and 1 day). Therefore, any given date advances by 1 day of the week each non-leap year, and by 2 days following a leap year.