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.