My therapists acknowledged holidays just like most people in general in everyday life. Sometimes yes other times no. They usually offered greetings when I mentioned something about the holidays first, I think. For me it's not really important but if it were, I would probably mention it to the T alongside my reaction. I think there are many people who do not observe a specific holiday or have some negative memories associated with it so I imagine many Ts would be reluctant to bring it up first.