Sunday comes first in order in calendars shown in the table below. In the Abrahamic tradition, the first day of the week is Sunday. Biblical Sabbath (corresponding to Saturday) is when God rested from six-day Creation, making the day following the Sabbath the first day of the week (corresponding to Sunday). Seventh-day Sabbaths were sanctified for celebration and rest. After the week was adopted in early Christianity, Sunday remained the first day of the week, but also gradually displaced Saturday as the day of celebration and rest, being considered the Lord's Day.

https://en.m.wikipedia.org/wiki/Names_of_the_days_of_the_week#Days_numbered_from_Saturday

So in Abrahamic religions, first day of week is Sunday, as the day after Sabbath. In Germany, Monday became day one in 1969 (DRG), and 1975 (FRG), respectively.