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What Are Week Numbers?

Most of us have learned that a year has 52 weeks. So why do some calendars have a week numbered 53?

Woman reading a notebook calendar

Weeks numbering varies in different countries.

© iStockphoto.com/AndreyPopov

52 Weeks Plus 1 Day

The reason is that a common year in our Gregorian calendar consists of 52 weeks plus 1 day, while a leap year has 52 weeks plus 2 days.

However, countries vary on exactly how they present and number the weeks in their calendars.

Monday or Sunday First?

There are at least six different week numbering systems currently in use around the world.

The most common is the International Standard ISO 8601, where Monday is the first day of the week followed by Tuesday, Wednesday, Thursday, Friday, Saturday, and with Sunday as the seventh and final day. Using ISO 8601 can help diminish uncertainty and confusion when communicating internationally.

Not ISO 8601 in US & Canada

About half the world's population do not follow the ISO standard. Countries like the United States, Canada, Australia, and New Zealand, put Sunday at the start of the week on their calendars.

Which Years Have 53 Weeks?

The weeks of the year in a Gregorian calendar are numbered from week 1 to week 52 or 53, depending on several varying factors. Most years have 52 weeks, but if the year starts on a Thursday or is a leap year that starts on a Wednesday, that particular year will have 53 numbered weeks. These week numbers are commonly used in some European and Asian countries; but not so much in the United States.

When Is the Weekend?

As the first day of the week varies in different cultures, so does the weekend. The Christian part of the world marks Sunday as their day of rest and worship, while Muslims refer to Friday as their day of rest and prayer. The Jewish calendar counts Saturday as the Sabbath—the day of rest and worship.

Some countries use a separate color for the weekends and reserve the black or gray for the weekdays.

Babylonian Origins

There are many different opinions as to how the seven-day week originated, but the most common explanation is that it seems to have originated when Babylonian astrologers assigned planet gods to the days of the week around 700 BCE. The Romans later replaced these names with their own planet gods, and Germanic and Norse people later did the same with some of their gods.

Topics: Weekdays