The idea here I think, is that as a non-christian I do not celebrate the birth of christ... that's the plain simple truth.
HOWEVER, the timing of the season is important. Mid winter is a much needed time to reconnect to family, home, and recharge the internalness of being human just stepping away from work for a couple of days and celebrating helps kick the hell out of winter blues. (I live in Canada - winters are [used to be anyway] long and grey, dreary and cold - and severely lacking in Vitamin D) Psychologically this mid winter holiday season is quite simply a requirement.
So for me, as an agnostic, the winter holiday season is celebrated with lights, feasts, family, friends, gifts, super cheesy decorations, snowmen, warm cocoa, wine and egg nog. If I do pull 'traditions' out of a beat up old travel trunk.. they are chosen for the sense of happy memories everyone shares. You'll find as many manger scenes in my home as you'd find of any other winter festival myth including santa, poseidon, sol, mithra... but I think the hogfather is my favouritest.
The bee in most secular bonnets is this winter festival 'ownership' by christians that while it's relatively new is very domineering and a little close minded at excluding what the season really is. The whole "Remember the true meaning of Christmas" mantra simply gets a little tiring when most of the people scowling it out are in fact forgetting the original meaning/reason of the season. That regardless of which myth you celebrate - it fills a human need during a difficult season to get together, kick up our heels and just be joyful in the very face of bitter cold ice and snow.. here anyway... there's nothing better than flipping hardship the joyful bird.