Dear Countdown Companions:This being the Monday after the first Sunday of Advent means that time, like that ever-flowing stream, will bear us all to the inevitable December 25 when we will proclaim Christ’s birth (and wonder why we put out so many decorations when it was warmer, since now it's way too cold to put them away).In anticipation, our world organizes itself to tick off the moments until that particular day—Christmas. It's very tough for us as North Americans to imagine Mary and Joseph trudging to Bethlehem in anything but, as Christina Rossetti memorably described, "the bleak midwinter," surrounded by "snow on snow on snow." To us, Christmas and December are inseparable. But for the first three centuries of Christianity, Christmas wasn't in December—or on the calendar at all.