Encyclopedia of the History of the Invention and Technology, Facts on File

Days of the Week

 While day and night are astronomical events, and the month is usually based on the revolution of the moon, the week is an entirely artificial division of time. Even the definition of week, the only commonly used period of time, is problematic. It is incorrect to define a ‘week’ as a 7-day period. It is more appropriate to define a “week” as an interval is greater than a day and smaller than a month.

A rapid survey shows that a week has different number of days for people from cultures from around the world. Ancient Romans, for instance, used nundinal cycle marked A to H on the Roman calendar. However, Romans counted inclusively and they included market day on every ninth day. Hence the Roman market day fell on “nundinae,” the ninth day. The ancient Egyptians used a 10-day “week,” as did the French Revolutionary calendar in the 1790s. A “week” in Mayan calendar had either 13 days or 20-days depending upon the type of calendar in use. The two differential lengths of the week were used in Tzolkin (divine) calendar.  The Tzolkin date was a combination of two “week” lengths.(Only Partial entries provided)