The seven-day week as we understand it was, if not originated, at least definitively justified by the Jews. Genesis 2:3 points out that the seventh day was the day on which God rested from creation, and the Law of Moses mandates that the seventh day is to be a day of rest for all - including slaves and even animals (Deuteronomy 5:13-14). In Jewish tradition, Saturday is the Sabbath, and certain Christian bodies keep this strict seventh-day sabbatarianism (notably the Seventh-Day Adventists).

But (as Christians believe) Christ was raised on the first day of the week (Matthew 28:1; Mark 16:1-2; Luke 24:1; John 20:1), after the Sabbath was over. Because of this, Christians traditionally have their primary worship events on Sunday, and the Mosaic sabbatarian regulations were gradually transferred to Sunday among the vast majority of Christians.

Nowadays, both Saturday and Sunday are seen as days of rest, and some calendars show Monday as the first day of the week since it's the first day of the "working week" (Monday-Friday only). You may be familiar with "seven to ten working days" which essentially means two weeks.