The regulative principle is often misunderstood. It's true generally speaking that Anglican and Lutheran denominations work on the principle of if it's not prohibited by the bible it's allowable while the Reformed tradition says if it's not in the bible it's not allowed. However, Presbyterians distinguish between matters of content or theological substance and matters of form in worship. The latter are not so important. Moreover, the principle was meant to exclude things like Roman Catholic saints days and Mariolatry, etc. Conservative Presbyterians have mounted good cases within their own tradition's beliefs for celebrating Christmas, and most Presbyterians do so. Christmas is celebrating the birth of the Saviour and that is certainly in Scripture. A related irony is that many who hold strictly to the regulative principle (even some who don't like Christmas) often celebrate Reformation Sunday.
But this is all getting a bit theological and drifting into areas that are verboten by the no religion thread police...
