Sabbath comes from the Hebrew word 'Shavat' (שבת) which means 'to stop' and it first appears in Genesis, as the day in which God completed the creation of Earth, and rested.
Following this example, the Jews were instructed to do nothing on the seventh day, but to worship God.
Those who did work on the Sabbath were facing excommunication and even execution (which of course, was carried out on a week day.)
Such harsh punishments were used, throughout history and to this day, to force people to do things they otherwise would not willingly do (such as pay taxes, desert the battle-field, etc.)
So it is curious to note that not keeping the Sabbath was considered to be a mortal sin. It would be the equivalent of nowadays imprisoning someone for not going abroad for the summer holiday.
The only feasible explanation seems to be that people were then forced to spend much of their 'free' day worshipping God.
Other religions were quick to adapt this concept (though choosing different days as their Holy Sabbath, to demonstrate the righteousness of their respective Gods) During those weekly forced visits to that variety of Houses of Worship people were numbed with psalms and then exposed to brainwash, via sermons.