people used something to name a period of time. and a period from 00:00 AM, til the next 00:00 AM we call it a day. then how about a period of 7 days? we name it week. The seven days also has its meaning of its name. here are the list of the meaning of name of the days.
Sunday - meaning Day of the Sun
Monday - meaning Day of the Moon
Tuesday - Day of the war god Tiw or Day of Mars
Wednesday - Day of Mercury
Thursday - Thor's day
Friday - Freya's day
Saturday -Saturn's day