You don't have to wait until January 1st. If you are going to smog check your Cadillac Sedan Deville before Dec 31, you may visit a Test Only or Gold Shield smog station.
Since the DMV sends registration renewal notices 60-90 days in advance they have already started requesting STAR smog checks for vehicle registration renewal notices which are due after the new year.
In the interim (before Jan 1st, 2013) you may visit a Test Only or Gold Shield smog station whenever the DMV requests a STAR certified smog check; just as if they had asked you to smog check your Cadillac at Test Only or Gold Shield Station.