Maybe there was a loss in the family you haven't quite gotten over. Maybe you've never celebrated the holidays like you'd like to because of a job, etc. Maybe there's simply built up frustration over being expected to celebrate the holidays when you don't feel like it. Maybe you're physically ill and the holidays don't matter this year?
Whatever the reason, maybe you just need someone to talk to. Help is only an e-mail away.
If you just don't feel quite right, but don't know why, maybe a brainstorm would help. Think of all the ways you are feeling or all of the things you are thinking about. Do this for 5 minutes. Then analyze what you wrote down might mean. That could help you at least understand.
Maybe you know what it is. If it was a loss, maybe writing that loved one a letter could help. Maybe screaming out loud will help. Possibly a good cry will help. I don't know what will help, but maybe you do.
This could be a time of peace and happiness. Will you allow it to be? If you need help with that, you know where to come.