All of the below answers are true. To summarize, the 7-day week has its origins in the story of Creation in Genesis, in the Hebrew Bible, also called the Old Testament. Jewish people celebrate this on Saturday. The early Church had two or three reasons for changing the day of the sabbath (day of rest and worship) to Sunday:
- to differentiate themselves from the Jews, possibly including both allowing Jewish Christians to practice on both days, and to allow all Christians (Jewish or not) to practice together on the same day.
- to honor the resurrection of Jesus, which occurred on a Sunday.
- and possibly in relation to replacing a Roman weekly festival with a Christian one, though that would have been about 200 years later, in 321AD.