It is a Christian holy day, set aside for rest, in remembrance of God's resting on the seventh day of creation. Originally, this was the Jewish Sabbath, recognized on a Saturday(the last and seventh day of the week). After the death of Christ, He lay in the tomb three days, and was resurrected on the third, which was a Sunday. That is why many Christians observe the day of rest and worship on Sunday. Most of western civilization was founded or influenced by Christianity, so the observance has become traditional, even for those that don't understand it.