The absolute earliest is the 1st of December, but probably not for another week after that, the 8th being a very appropriate date, given the feast day it is. It should finish on the 6th of January. Funnily enough, the time of year that you are least likely to hear Christmas music is during Christmas. For weeks before Christmas you hear Christmas music, including lots of versions of "The 12 Days of Christmas" and then it all stops a couple of days into it. The earliest you should ask someone "How was your Christmas?" is on the 7th of January. It is obviously too early before that.