Why does the days of the week begin with Sunday?

In The Bible, in the Old Testament book of Genesis, it says God rested, after having created everything in six days, on the Sabbath, which was Saturday. That is where the idea of having Saturday as the seventh day originated, which would make Sunday the first day. Of course the names of the days were not the same in Hebrew as the ones we use now in English. Much later, in the Middle Ages, the only church that was legal was the Roman Catholic church. That church made the decision to start the days of the week with Sunday, because their day of worship was Sunday. It has remained the same since then, at least in European and English-speaking countries.