Hello! Thank you for your question. I would be happing to give you some suggestions based on points of etiquette for this situation.
In general, there is no need to regard most invitations as a command. For the most part, you can feel at liberty to decline or change your mind. You do not need to feel compelled to give a detailed excuse, just be polite and extend your regrets if questioned. If you want to skip the party, go ahead; skip it, and don’t feel guilty if you do. You can tell whomever asks that you have a conflict, a family obligation, or just that something that came up. People have myriad family and social obligations at this time of year, anyway. It is a good idea to keep such excuses short and simple. Making up an imaginary excuse has a high potential for backfiring.
However, be aware that in some offices, attending these things borders on being mandatory. If that is the case in your office, you might want to suck it up and make an appearance. You can look at it as a work obligation like a mandatory meeting. Then you can just put in an appearance by arriving late and leaving early. In addition, you might want to consider looking for a job that is more in keeping with a place and people with whom you would like to spend social time.
An interesting thread regarding this question can give you further perspectives on this question. It is available here: http://ask.metafilter.com/194153/Miss-Manners-in-the-workplace
Please rate this response so that I can get credit for my time. Then you could feel free to ask any follow up questions you might have. Best regarsds and happy holidays to you, Patience P.