Before the Industrial Revolution began to loom, the week consisted of six days of work and one day of rest, as described in the Bible.
The official reason for Sunday as a day of rest for Christians is the excuse of commemorating the Resurrection. “Sunday is the new Sabbath” was basically the slogan with which in the fourth century, with a fair share of anti-Semitism, Christianity codified Sunday as a day of rest in civil and ecclesiastical legislation, even though ‘Sabbath’ comes from the Latin sabbatum and this from the Hebrew šabbāth (‘day of rest’), which in turn derives from šābath (‘to rest’). Several centuries later Islam would determine that “Friday is the new Sunday”, so that no one would be confused.