This article is from the Calendars FAQ, by Claus Tondering firstname.lastname@example.org with numerous contributions by others.
For the Jews, the Sabbath (Saturday) is the day of rest and
worship. On this day God rested after creating the world.
Most Christians have made Sunday their day of rest and worship,
because Jesus rose from the dead on a Sunday.
Muslims use Friday as their day of rest and worship. The Qur'an
calls Friday a holy day, the "king of days".