I work 3rd shift and my employer changed our normal start time for christmas night to avoid paying us time and a half.... can they do this.

YOur post is unclear, but if you are saying either of the two things your post could mean, the answer is yes, they can. First of all, your employer is not required to pay you "holiday pay" at all, if by holiday pay you mean a premium for working on the holiday. So it is legal for them not to pay you time and a a a half for working on the holiday.

Furthermore, if you are talking about changing your start time so that you do not work more than 40 hours in a week, that is always legal, holiday or not.

If you mean neither of these things, please clarify.