In fact, the practice dates back further than you might suspect, ans has nothing to do with devaluing the Christian festival, as many people believe. In reality, both Christ and Christmas have been abbreviated for at least 1,000 years. The word Christ appears in Medieval documents as both 'XP' and 'Xt' and can even be found in this form in the Anglo-Saxon Chronicle from 1021. By why were those particular letters used?
To find out more you'll have to pick up my book Christmas Explained: Robins, Kings and Brussel Sprouts!