Day Of The Week Calculator

This “Day of the Week Calculator” is tailored for genealogists and family history. It gives you the specific day of the week for any given date in history. To use the tool:

  1. Provide any date since 1753.
  2. Click the “Find Day” button.

Day of the week:

Why Find The Day Of The Week?

This tool can give you a better understanding of historical records and family events. Here are some ideas to get you started with getting calendar context...

Discovering that an ancestor was born, married, or passed away on a particular day can provide greater insights into the event. For example, weddings on weekdays might suggest a small, intimate ceremony, while those on weekends could indicate larger gatherings.

Knowing an ancestor enlisted in the army on a specific day might coincide with a notable event in a war or conflict.

Historical documents or oral family histories can have discrepancies. Determining the day of the week can help validate the accuracy of the dates.

Why Only Dates Since 1752?

We set the earliest cut-off at 1752, which isn't all that long ago in genealogical terms. Let me explain the challenge!

The reason is the historical shift in countries moving from the Julian Calendar to the newer Gregorian Calendar. The Julian Calendar is about ten to fourteen days different to the Gregorian calendar.

If you want to know more, check out our calculator that converts from Julian to Gregorian!

But if the calendar shift was the only difference, it would be easy enough to shift the calculations to find the day of the week. The real problem is different countries were on the Julian calendar at different times.

So, the same day in neighboring countries could be a different date and day of the week.

Most English-speaking countries made the shift on the 15th of September 1752. That includes the United Kingdom and its former colonies like the United States.

How Does It Work?

Our calculator is based on Zeller’s congruence.

Julius Zeller was a German reverend and mathematician who came up with a formula to calculate the day of the week in the 19th century. He presented his method to the prestigious French society for mathematicians in 1883.

His method is known now as Zeller’s congruence.

I won’t explain it here in detail, but it’s a method of using some fairly simple algebra to calculate the day of the week for any date after the adoption of the Gregorian Calendar.

Here is the first part of the formula, to give you an idea of what it looks like:

Zeller's Congruence for Day of the Week

There are a few wrinkles such as assuming that years start in March and treating January and February as if they are in the previous years.

If you follow along the worked examples in this old tutorial, you’ll see how it works.

How Does The Tool Deal With A Leap Year?

We must account for leap years because they introduce an extra day, February 29th, into the calendar. This extra day affects the day of the week for dates following February 29th in a leap year.

Here are the basic rules for leap years:

  1. Every year divisible by 4 is a leap year.
  2. However, years divisible by 100 are NOT leap years, unless...
  3. The year is also divisible by 400. So, for example, the year 2000 was a leap year, but 1900 was not.

Start And End Day Of The Week

The concept of the "start" and "end" of the week depends on cultural, religious, and regional background. Here are some widely adopted standards.

United States and Canada

In North America and other countries, the calendar week typically starts on Sunday and ends on Saturday. This also includes South America, the Philippines, China and Japan.

This is the default for many U.S.-based software systems and services unless they adjust for international audiences.

International Standard (ISO 8601)

This standard considers Monday as the start of the week and Sunday as the end.

Many countries, especially in Europe, follow this standard in everyday contexts as well.

Middle East and Muslim Countries

In countries with a predominantly Muslim population, the week often starts on Friday and ends on Thursday. Friday is a day of congregational prayer and rest.

Jewish Calendar

In the Jewish calendar, the week starts at sunset on Friday. It ends at nightfall on the following Friday.

Related Links: More Date Calculators

I've mentioned our Gregorian calculator, but we also have more date calculators. Here are a few:

You can find all our genealogy tools and calculators listed here.