Day of Week Calculator — What Day Was That Date?

How the Calculation Works

The calculator uses JavaScript's built-in Date object, which implements the Gregorian calendar. The getDay method returns a number from 0 (Sunday) to 6 (Saturday), which the calculator converts to the day name.

This is mathematically equivalent to Zeller's Congruence, a formula developed in 1882 by Christian Zeller. The formula uses the month, day, and year to compute the day of the week directly without iterating through the calendar.

For any date after October 15, 1582, when Pope Gregory XIII introduced the Gregorian calendar, the result is accurate to the day. For dates before that, the calculator uses Gregorian rules even though most of Europe used the Julian calendar at the time. Historical records from before 1582 may show different days of the week.

Famous Dates and Their Days

• July 4, 1776. The Declaration of Independence was signed on a Thursday.
• November 22, 1963. President John F. Kennedy was assassinated on a Friday.
• July 20, 1969. Apollo 11 landed on the Moon on a Sunday. Neil Armstrong stepped onto the lunar surface at approximately 10:56 PM Eastern Time.
• September 11, 2001. The attacks on the World Trade Center occurred on a Tuesday.
• January 20, 2009. Barack Obama was inaugurated as the 44th US President on a Tuesday.
• January 1, 2000. The new millennium began on a Saturday.
• January 1, 2100. The next century will begin on a Friday.

Why People Use This Calculator

• Birthday checks. What day was I born on is the single most common reason. Many people share their day on social media for milestone birthdays.
• Wedding planning. Check what day of the week a date falls on before committing to a venue. Most weddings are on Saturdays. Friday and Sunday weddings are growing in popularity for cost savings.
• Historical research. Genealogists, historians, and students use the calculator to verify dates in old documents. The day of the week can confirm or contradict claims in historical sources.
• Future planning. Check the day of the week for any future date before booking events, vacations, or important meetings.
• Recurring schedules. Verify that the first Tuesday of next month or the third Friday of March is what you actually mean.
• Pattern recognition. Friday the 13th occurs at least once every calendar year.

Day-of-Week Patterns

The Gregorian calendar repeats every 400 years exactly. Within that 400-year cycle, the average year is 365.2425 days long.

• January 1 falls on a Tuesday 56 times per 400 years, which is roughly every 7 years on average.
• The 13th falls on a Friday slightly more often than any other day of the week, 688 out of 4,800 possible 13ths per 400 years.
• A year that starts on Sunday has a Friday the 13th in January.

If your birthday is on a fixed date, it falls on the same day of the week every 6, 5, 6, or 11 years depending on leap year placement. Over a 28-year cycle in a non-century span, each date falls on each day of the week 4 times.