A long long time ago there was no concept of week or weekdays. An year was divided into 12 months of 30 days. That’s it! Then slowly ancient people felt the need to fix certain days for certain activities. In the beginning, they experimented with making a 10 day week and also a 5 day week. But finally they adopted a seven day system. Why did they do that? Mostly because at that time ancient people were aware of seven heavenly bodies in the sky – Sun, Moon, Mars, Mercury, Venus, Jupiter and Saturn.

The names for the days of the week are also somewhere related to these planets.

Sunday – Sun is the source of all energy and thus the first day of the week was named after the most important heavenly body – The Sun.
Monday – was named after The Moon. Since it is a moon’s day it was called Moonday. Somtime later it changed to Monday.
Tuesday – ‘Tiw’ was the God that was associated with planet Mars. Thus ‘Tiwsday’ was the day of Mars.
Wednesday – ‘Woden’ was the God that was related with Mercury in olden times. Thus the name ‘Wodensday’.
Thursday – named after ‘Thor’ that was often referred to as God of Jupiter.
Friday – Comes from ‘Friggs’day’ that meant day of Venus.
Saturday – was the day of Saturn.

Had ancient people discovered all nine planets at that time, we would probably be following a nine day week !! Thank God that did not happen, otherwise we would be going to school for 7 days a week and not 5 days.