The 2nd commandment: Thou shall have no Graven Images
The star on the tree represents Christ and Christmas is suppoused to be his birthday.
Not only does this break the 2nd commandment, but it also breaks the 1st and 9th.
1st because you are worshiping a flippin' star over God
and the 9th because Jesus doesn't have a "Birthday" because he doesn't abide by time. God and Christ have always existed.