As far as Christmas and Easter goes, the first of these specifically has a non-religious explanation in HPMoR:
The atmosphere at Hogwarts before Yuletide was usually bright and cheerful. The Great Hall had already been decorated in green and red, after a Slytherin and a Gryffindor whose Yule wedding had become a symbol of friendship transcending Houses and allegiances, a tradition almost as ancient as Hogwarts itself and which had even spread to Muggle countries.
No similar explanation has been given for Easter, but I think it’s reasonable to suppose that one exists.
Christmas and Easter both borrow heavily from pre-Christian European traditions. Presumably those threads are carried over even more strongly than in muggle Europe.
As far as Christmas and Easter goes, the first of these specifically has a non-religious explanation in HPMoR:
No similar explanation has been given for Easter, but I think it’s reasonable to suppose that one exists.
I took that passage to indicate the tradition of green and red colors during Christmastide, not of the origination of any holiday,
Christmas and Easter both borrow heavily from pre-Christian European traditions. Presumably those threads are carried over even more strongly than in muggle Europe.
That does sounds like a solid theory as to why Wizards celebrate those holidays. I’ll update my beliefs with this new evidence.