ISO 8601 and Weekday Calculation

The International Organization for Standardization (ISO) governs ISO 8601, the global benchmark for representing dates and times. Although widely implemented—especially in developed regions—not every country follows this standard. ISO 8601 also standardizes the method for indicating the day of the week, relying on an algorithm to determine the specific weekday for any given month and year. This algorithm, known as Zeller’s congruence, was developed by Christian Zeller. According to ISO 8601, Monday is recognized as the first day of the week.

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The Gregorian Calendar

Named after Pope Gregory XIII, the Gregorian Calendar reflects the time periods based on celestial motions: roughly one month for the moon’s cycle around the Earth and about one year for the Sun’s cycle. In ancient times, the regularity of these celestial bodies made them natural markers for measuring time. Over time, these observations were refined into the modern Gregorian Calendar that is widely used today.

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Origins of the Names of the Days

Most day names trace their origins back to Roman deities, a legacy stemming from the Hellenistic period (spanning from the death of Alexander the Great in 323 BC to the emergence of the Roman Empire in 31 BC). For example, in the Roman system, Sunday was associated with Sol, the sun god, while Saturday was linked to Saturnus, the god symbolizing wealth and liberation. Other days have equally fascinating origins:

• Monday derives from Luna, the personification of the moon.
• Tuesday is named after Mars.
• Wednesday comes from Mercurius (Mercury).
• Thursday is linked to Jove (Jupiter).
• Friday traces its name to Venus.

These Latin names spread through various cultures and languages, influencing not only European tongues like French and German but also languages in Asia and India. For instance, the Japanese word for Friday—formed from “kinsei” (Venus) and “yobi” (day)—and the Indian term for Thursday, “Guruvara,” referencing Brhaspati (Jupiter’s guru), exemplify this global cultural diffusion.

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Day-by-Day Trivia

Monday

• Statistically, it is the day when the U.S. stock market is most likely to close higher than it opens.
• It is unique in that its name can be rearranged to form a single other word.
• A 2011 study noted that people tend to moan for about 34 minutes on Mondays—more than on any other day.
• People generally weigh the most on Monday.
• Due to a higher incidence of suicides, Monday is sometimes referred to as “suicide day.”

Tuesday

• Many workers find Tuesday to be their most productive day.
• It is a common day for submitting job applications.
• Historically, Tuesday is remembered for Black Tuesday (October 29, 1929), the day of the infamous stock market crash that triggered the Great Depression.

Wednesday

• Often called “Hump Day” in the U.S., Wednesday marks the midpoint of the workweek.
• While Ash Wednesday is traditionally a day of prayer and fasting in Christianity, it is also the name given to a series of devastating bushfires in southeastern Australia in 1983, which resulted in 75 deaths and over $300 million in damages.

Thursday

• In the Christian calendar, Maundy Thursday (or Holy Thursday) commemorates the Last Supper and occurs the week before Easter.
• In contemporary culture, “Thirsty Thursday” is a term used by students and young professionals to signal the start of early weekend celebrations involving drinks.

Friday

• When the 13th of the month falls on a Friday, it is often considered unlucky, though some cultures see it as a lucky day.
• In the United States, Black Friday—occurring the day after Thanksgiving—marks the beginning of the Christmas shopping season, noted for its massive sales. However, this day has also been associated with incidents of extreme retail rush, including a tragic event in 2008 in New York.
• Good Friday, observed the Friday before Easter, commemorates the crucifixion of Jesus.

Saturday

• In Israel, Saturday is observed as a day of rest, with most businesses and public transport ceasing operations.
• Australia and New Zealand traditionally hold elections on Saturdays.
• In Sweden, children are traditionally allowed to enjoy candy only on Saturdays.
• There is an old belief that Saturdays are the only suitable days for hunting vampires because folklore suggests that they remain in their coffins on this day.

Sunday

• In some regions, like parts of the Middle East, Sunday is considered the first day of the week rather than Monday.
• For many Christians, Sunday is a day dedicated to rest and worship, often referred to as the Sabbath or God’s rest day.
• Around the world, banks are generally closed on Sundays.
• If a month begins on a Sunday, it is likely to include a Friday the 13th.
• In the United States, Super Bowl Sunday, featuring the championship NFL game, is one of the biggest sporting events of the year.