|Aside from the 'company' party, there are numerous local parties on (usually) Dec 24th - in my group, it is put on by a group who are Buddhists.
It's really a holiday party, rather than a christmas party. This is not due to "PC" fears, but rather to what this time of year is (at least in the urban parts of the US) - the holiday season.
We're together - having a good time - the corn, the wine, and the oil of plenty - and doing it together. That is the cause for celebration. Those in your office who are not christians - that's what they celebrate with you. This short period when people are nicer to one another (even to strangers) Were these people to debate this (with you), it would certainly be considered bad taste on their part (Why did they come? is then overheard). There are many times when minorities have learned to smile and nod their heads in agreement. They are celebrating with you - perhaps even for you and your joy - but celebrating christmas, per se? Hmmmm . . .
The above is just opinion; what follows, however, is food for thought. All the complaints about Political Correctness - how disingenuous! It is (almost invariably) the words of those who are in ownership of the expression that was changed. Were that shoe on the other foot - perhaps they'd want a little consideration for their sensibilities.*
It's really a matter of considering the context: if you send holiday cards, is it not inconsiderate to send a non-christian a deeply religious christian theme? The question, really, is how does one take into account other's sensibilities?
Unfortunately, one can always find absurd 'PC' examples and use them to try discredit the concept of considering the feelings of others . . . by giving them a label. For a christian church to avoid calling its party a 'christmas party' would be absurd. For a municipality with a diversity of citizens, who are equal partners, it really does make sense. Context. Context. Context.
There's a Native American saying: 'Let me not criticize another man until I have walked a mile in his moccasins'.
A favorite theme in Mid-America: Put prayer back in schools. These half-wits think that the prayer will be to their liking - but what if the teacher leads in a daily praise of satan?