I can think of two possible justifications.
The first is that society wants to promote particular ideas, values, individuals, or the like; think of the 4th of July or President's Day. I find this defense problematic: the choice of holidays is a vehicle for thought control.
The second justification asserts that legal holidays solve a co-ordination problem by helping people take vacations at the same time as their friends and relatives. This view has some merit, but a countervailing effect is that promoting specific vacation days generates crowding at airports, beaches, and so on.
So I do not see a convincing justification for government holidays, especially not those associated with religion.
Yes, I know I am a Scrooge. My family has been telling me this for years.