Perpetual Calendar

Use the form below to find the day of the week for any date from January 1, 1 B.C. to December 31, 2999. It isn’t actually a true perpetual calendar, since it only covers 3,000 years.

The form uses JavaScript but does not use cookies. All of the calculating is done by your own computer; the results will appear instantly in a pop-up alert box.

Be sure to read the notes below if “Old Style” and “New Style” appear in the results. The formula used for this script is also explained on this page.

Choose a date, then find the day of the week

in the Year

Old Style and New Style

One year is equal to 365 days, 5 hours, 48 minutes and 46 seconds. The ancients were unaware of this, but they got it pretty close, at 365 and a quarter days. So the Julian Calendar (named for Julius Caesar) was devised with an extra day every four years, at the end of February (we know this as Leap Year).

The minor difference of 11 minutes and 14 seconds doesn’t sound like much, but it adds up to a full day about every 128 years. Over the centuries, the calendar drifted off by 10 days and needed to be corrected. Pope Gregory decreed that the day following October 4, 1582 would be October 15 (skipping 10 days), and that even century years would only be leap years if divisible by 400. In other words, 1600 and 2000 would be leap years, but 1700, 1800 and 1900 would not. This is the Gregorian Calendar, which we still use today.

Most countries converted to the Gregorian Calendar either immediately or within a few years, but Britain and the American Colonies did not convert until an act of Parliament in 1752 fixed the day following September 2 as September 14 (skipping 11 days, since 1700 had been observed as Leap Year, making the discrepancy worse).

The old reckoning became known as Old Style, and the corrected reckoning is called New Style. So even though the days of the week were observed the same around the world, the calendar dates were different in different countries for about 170 years. Our form will yield results for both Old Style and New Style dates during the transition period.

But wait—there’s even more! Britain observed the start of the year as March 25, from sometime in the 12th Century until the conversion to Gregorian in 1752. This pertained mainly to the church calendar; the civil calendar always started the new year on January 1, as we do today.

This meant that someone born on February 7, 1546 on the civil calendar, would be considered as born in February 7, 1545 on the church calendar. Because of this discrepancy, and to avoid ambiguity, historians have used the notation: 7 February 1545/6.

OUR FORM ALWAYS USES JANUARY 1 AS THE START OF THE NEW YEAR. Please be sure you are using the correct year, if the date is in Old Style reckoning and it falls between January 1 and March 24. If a date is notated as 1545/6, please use 1546 in our form.

The Formula

It isn’t very difficult to figure the day of the week for any date, using a simple formula which I will describe here.

Let’s suppose we want the day of the week for July 4, 1776, when the American Declaration of Independence was signed.

First, get the number corresponding to the month, from this chart:

January1 (0 if leap year)
February4 (3 if leap year)

OK, so we have a zero for July.

To that, add the day of the month. For July 4, of course that would be four.

Now for the year, use the last two digits (76 in our example). Divide by four, dropping the fraction; then add what you started with (76 divided by four is 19; 76 plus 19 equals 95).

Now add for the century (New Style, or Gregorian Calendar):

1700s, 2100s, 2500s4
1800s, 2200s, 2600s2
1900s, 2300s, 2700s0
2000s, 2400s, 2800s6

Don’t use the 18th Century, since that includes 1800; just do the 1700s. Our number is four.

If the date is Old Style (Julian Calendar), subtract the “hundreds” from 18 (1611 would yield a 2).

Now add everything together, divide by seven and take the remainder. In our example, we have zero for July, four for the 4th, 95 from the year and four for the century. That adds up to 103; dividing by seven gives 14 (which we disregard) and a remainder of five, which we use to get the day of the week thus:


Five corresponds to Thursday, so July 4, 1776 fell on a Thursday. You can verify this by using the form on this page.

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