Christianity gets its seven day period from Judaism, and it may well hearken back to Babylonian astronomy (seven planets, including the sun (Sunday) and moon (Monday)). The Romans, on the other hand, had an eight-day week with market days every eighth day (ninth counting inclusively, hence their name nundinae). In the middle ages, market towns had market days on fixed days of the week, every week, providing a commercial rhythm that had to give some rise to weekly consciousness before the twentieth century.

while the seven day week is arbitrary, it is also worth noting that the French Revolution had experimented with a metric ten day week, which ended up being a total bust because laborers only got one week in ten of rest rather than one in seven.