I generally act the same way throughout the year. I enjoy seeing the happier faces this time of year but generally only say Merry Christmas because I assume that's what everyone expects to hear. I am kind and curtious all year, always saying "please", "thank you", "you're welcome", "have a nice day", and offering a genuine smile.
I think the "general" public has a tendency to be extra grouchy this time of year, while they also have a tendency to be extra happy/polite.