Christmas is an inherently syncretic holiday and perfectly appropriate for UUs to celebrate. Puritans in New England forbade celebrating it because of its pagan/solstice celebration roots. There's nothing in the Bible that says Jesus was born in December, and some scholarship that points to July (if you believe the tax motivation in Luke).
We have an evening candlelight service that is one of the largest attendances of the year. There are many people who are uncomfortable with orthodox theology that still feel the longings of transcendence that Christmas used to symbolize for them, and UU churches offer people a chance to experience that without feeling like hypocrites.
Too bad they only come once a year.
On the other hand, a few people get ticked off that our hymnal changes the lyrics (a little) to a few well-known Christmas carols, e.g.:
"O come, O come, Emmanuel, and
with your captive children dwell
....Rejoice, rejoice, Emmanuel shall come
within as Love to dwell"
(instead of
ransom captive Israel and
come to thee O Israel)
