God rested on the seventh day, which was the Sabbath, so Saturday.

After the resurrection of Jesus, which was on a Sunday, Sunday became an important day for Christianity. This was the reason why the Roman Emperor Constantine made Sunday a resting day. It was still the first day of the week, though.

In the 20th century, it was decided (only for secular/economic reasons) to keep the weekend, Saturday and Sunday, as a unit and call it the end of the week, making Monday the first day of the week.