"The Week" by David Henkin
“The Week” goes out of Henkin’s way to emphasize that weeks are not natural. Years have to do with the sun, days have to do with the sun but weeks are a human invention. I wondered if some societies have used more or less days than 7. I got this:
Yes, many historical and traditional societies have used cycles shorter or longer than seven days, such as the ancient Roman eight-day market cycle, the Egyptian and Chinese ten-day cycles, and the traditional West African four-day or five-day market weeks. [1, 2, 3, 4]
Ancient and Traditional Week Lengths
3-to-4 days: Isolated pockets in parts of central Africa historically operated on ultra-short three- or four-day intervals. [1]
4 days: The Igbo people of Nigeria traditionally organize time around a four-day market and ritual week. [1]
5 days: The Javanese people of Indonesia use a traditional five-day cycle called a pasaran, and various Korean and West African regions used five-day market structures. [1, 2, 3, 4]
6 days: The Akan people in Ghana use a six-day cycle alongside other reckonings. [1, 2]
8 days: Early Romans used an eight-day agricultural and market cycle known as the nundinae before switching permanently to the seven-day week. [1]
10 days: Ancient Egyptians and ancient China split their months into ten-day units (such as the Chinese xun). [1, 2, 3]
13 days: The Aztecs and other Mesoamerican cultures structured time using a 13-day cycle, often called a trecena. [1]
15 days: Certain older Chinese administrative systems divided months into two 15-day periods. [1]
Modern and State Experiments
10 days: During the French Revolution, the government replaced the seven-day week with a decimal ten-day décade to eliminate religious influences, though it was short-lived. [1]
5 and 6 days: The Soviet Union experimented with continuous rolling five-day and six-day workweeks in the 1920s and 1930s to keep factories running constantly, but abandoned the practice due to social complications. [1, 2]