This has to do with the 16 hour time difference between Melbourne, Australia and U.S. East Coast. That is a BIG time difference. But, it is still going to be incongruous having a Test match being played on Christmas day — even if it is at 7pm at night. That said, I will probably watch it.
As has been the case in recent years we will have two parallel Tests — the other the South Africa v England Test from South Africa. That will air on Boxing Day.