The Germans and the Romans invented the day system. The names used to describe the day of the week was taken from Hellenistic astrology and were named after the gods in folklore. While the Romans first introduced the system, the Germans substituted the Germanic deities for the Roman ones except for Saturday. This was called interpretatio germanica.
The timeline when this happened is not really known but historians estimate that it happened sometime after 200 AD but before the 6th and 7th century. The specific days of the week got their names from German languages, particularly West German. Then the names were changed to be named after different deities. Some cultures have the week beginning on Sunday and others have the beginning of the week starting on Monday.