In Pawn of Prophecy, by David Eddings, the winter holiday was called Yule. It mimicked every winter holiday celebration I can think of from Christmas to Hannukah to the Feast of St. Lucia. In Guy Gavriel Kay's, Tigana, the annual Ember Days of Autumn feast celebrates the deicide of the God Adaon, and mimics the Dionysus myth.
There--I gave you two. Now, I am very curious to see what you come up with. Winter, spring, summer or fall--or none of the above! What seasonal festivals, national holidays, or any other event of that sort can you tell me about whether one you've read, or written yourself.