Why 1 Week = 7 Days? Learn Astronomy

Why are we using seven days a week? Why not five days, reducing two working days? Or ten-day week, extending few more weekend days? Is it because seven is a prime number?

Calendars have been used as records of past events and as a tool to predict the future events. Ancient Egypt and Chinese had a 10-day week. USSR changed from the 7-day week to a 5-day week and then to 6-day week. The Igbo of Nigeria have a traditional calendar with a 4-day week. The Javanese people of Indonesia have a 5-day week. But seven-day week is used now all over the world. Historically there were three types of calendars followed namely, Lunar, Lunisolar and Solar.

Emperor Constantine introduced the seven-day week in the calendar in 321 AD. Before that there were three fixed dates the calends(1^st day of the month), the ides(the 15^th of few months) and the nones(the 13^th in other months). Constantine made Sunday as the first day of the week and set it aside as the Christian day of worship.

Two theories are attributed to the selection of seven day week. Some believe that it was based on the story of creation as narrated in the Book of Genesis. According to the Bible, the world was created in six days and the seventh day was a day of rest. Others believe that the number of days in a week was based on the number of celestial objects known at that time – Sun (Sunday), Moon (Monday), Mars (Tuesday), Mercury(Wednesday), Jupiter(Thursday), Venus(Friday) and Saturn(Saturday). The second theory seems to be more appropriate as the days are named after the above planets.