Genesis 2:2

"By the seventh day God completed His work which He had done, and He rested on the seventh day from all His work which He had done."  Genesis 2:2 NIV

God has completed creation and has dedicated the 7th day to rest, but why would God rest?  He certainly doesn't need rest.  He is God, after all.  Perhaps this is an example for mankind.  We have six days of labor and then the last one for rest.  Or maybe he's foreshadowing the end, after our labors on earth are done we go to him for eternal rest.

It's important to note, as both the Apologetics Press and the QBible state, that the Hebrew word for "rest" does not mean that God was tired or worn out, but it simply means to stop or desist whatever one was doing before.  So this verse is just saying that God stopped all his work on the 7th day.