Well, he didn't actually. You see, the First Story of Creation in the bible was written, or actually stolen by the Israelites while they were in Babylonian Captivity. Strangely enough the Biblical version is virtually the same as the Babylonian version which was written many years prior to the Israelite version. But there was one difference. The seventh day, god rested. Why did god rest?
I'll tell you why. The Israelites were in the harshest slavery they would ever see until the Nazis. In the story of creation, they were trying to ask, why god have you done this to us. Well actually Genesis to II Kings had this purpose, none the less, though, they wondered what did we do to lose the holy land. The answer in the story of creation was that he didn't take away the holy land. Rather the entire world was theirs. So even in captivity by the Babylonians, they were in the world god made for them. They could see god everywhere. So why the rest? Well god always demands something from people. God was really into sacrifices at the time. Problem is that the Israelites where in slavery and had nothing to slain for their god. So they gave god the only thing they had in their possession. Their life and time. So on the seventh day they rested. That's why god "rested". Also you can notice poetry in the first story of creation. Match day 1 to 4, 2 to 5, and 3 to 6.