The Arizona desert is wonderful at that time of the year. It will usually be sunny (hey, we have over 300 sunny days a year) but the nights are cool. For someone from Michigan, the daytime temperature is ideal. Since Christmas is on a Friday this year, I would say your best bet would be to fly on the 25th or 26th as most people will want to stay until Sunday the 26th. Those early morning flights are always the best because people oversleep and leave you that open seat you were praying for.