Day of the Week Finder – What Day Was July 4, 1776?

July 4, 1776, was a Thursday. On this day, the Continental Congress adopted the Declaration of Independence in Philadelphia, marking the birth of the United States of America. The day of the week is verified using the Gregorian calendar, which Great Britain and its American colonies had adopted in September 1752.

There are several methods to calculate the day of the week for any date. The most famous algorithm is Zeller's formula, which uses modular arithmetic. Other methods include the Doomsday algorithm (by John Conway), and simply using programming languages (like JavaScript's Date object). Our tool uses the built-in calendar calculations of modern browsers, which are accurate for dates from year 1 to 9999 under the proleptic Gregorian calendar.

Zeller's formula, published by Christian Zeller in 1882, calculates the day of the week for any date in the Gregorian or Julian calendar. The formula is:

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

Where q is the day, m is the month (3=March, 4=April, ..., 14=February), K is the year within the century, and J is the century. The result h maps to Saturday=0, Sunday=1, ..., Friday=6. It's elegant and still used in many software implementations today.

The primary reason is the calendar switch from the Julian to the Gregorian calendar. Different countries adopted the Gregorian calendar at different times — for example, Catholic countries in 1582, Great Britain in 1752, and Russia not until 1918. During the transition, several days were skipped. Our tool uses the proleptic Gregorian calendar (extending the Gregorian rules backwards), which is the standard for most modern date calculations. For dates before a country's official adoption of the Gregorian calendar, the historical weekday may differ by 10–13 days from our calculated result.

This tool uses JavaScript's native Date object, which is highly accurate for dates between year 1 and 9999 under the proleptic Gregorian calendar. It correctly handles leap years (including century leap-year rules) and month lengths. For dates outside this range or for Julian calendar conversions, specialized libraries or manual calculation methods (like Zeller's formula with Julian mode) are recommended.

The Julian calendar (introduced by Julius Caesar in 45 BC) has a leap year every 4 years, averaging 365.25 days per year. The Gregorian calendar (introduced by Pope Gregory XIII in 1582) refines this by skipping leap years in century years not divisible by 400, averaging 365.2425 days per year — much closer to the actual solar year of ~365.2422 days. The accumulated difference (about 3 days every 400 years) caused the Julian calendar to drift relative to the seasons, which the Gregorian reform corrected by skipping 10 days in 1582 (and more in later-adopting countries like Britain, which skipped 11 days in 1752).

The Doomsday algorithm, invented by mathematician John Conway, is a popular method for mental calculation. It assigns a "Doomsday" (the same weekday) to certain easy-to-remember dates each year (like 4/4, 6/6, 8/8, 10/10, 12/12, plus 7/11, 11/7, and the last day of February). Once you know the Doomsday for a given year, you can quickly figure out the weekday for any date by counting from the nearest Doomsday anchor. With practice, you can determine any date's weekday in seconds!

English weekday names come from a blend of Roman and Norse mythology. Sunday and Monday are named after the Sun and Moon. Tuesday comes from Týr (Norse god of war, equivalent to Mars). Wednesday is from Odin/Woden (Mercury). Thursday is from Thor (Jupiter). Friday is from Frigg/Freya (Venus). Saturday retains the Roman name from Saturn. This pattern mirrors the seven classical planets that ancient astrologers associated with each day.