You also have to remember that our 'day' 'month' and 'year' have little or no meaning with regards to alien species. So celebrating alien festivals will likely have to float around the calender each year.
Let's take christmas for example. In World of ******** all of the major cities are decorated, there's a santa and there are christmas-themed quests. City of Heroes has giant snowmen attacking the city for the heroes to stop. That just wouldn't fit into STO.
We won't decorate the hull of our ships for holidays like christmas, so space and space combat would look exactly the same.
Ground combat is usually in enemy military facilities or on strange planets. It wouldn't make sense suddenly seeing a christmas tree or other holiday decorations there.
Our home stations would probably not change that much either. Maybe stardock would get a single christmas tree, but that's it. And you wouldn't see people celebrating either. Those celebrating are off duty, and celebrating back on earth with their loved ones. If you are on duty at that time, you better look as professional as on any other day.
It seems like a waste of energy to have special attention to holidays, when they can go almost unnoticed in game.
That being said, it would be nice to have something extra when we're home during a holiday. But it doesn't have to be linked to the holiday in any way. Maybe a nice borg invasion to keep us busy during christmas, or an undine fortress-ship roaming space during the summer holidays.