If you want to celebrate something, I suggest celebrating the Winter Solstice, as it's a natural, scientific phenomena close in time to Christmas and allows for many of the same basic concepts: crafts, family, evergreens, light, firelight, candles, the sun, small treasures that have been hidden away to ease the darkness of the long winter. It can be done "pagan," but it can also be done as humanist/scientist/fact-based. Likewise Summer Solstice, and both equinoxes. These are based on the earth's passage around the sun which causes seasons. The seasons have been celebrated in some fashion since humankind began, probably, and winter quite heavily, because it's a hard season for humans to get through, from ancient peoples to George Washington and his men at Valley Forge, to modern day.