1) Christmas time comes around. You are an Atheist. You wake up on December 25 to presents for you under the tree, and realise what Christmas is all about; The birth of Jesus. You feel guilty that you still get presents on Christmas, though you don't believe in it.
2) Last year, when you were 8 years old, you parents broke it down to you: There is no Santa Clause. It's Chrismas morning today, and you've got lots of presents under the tree! You remember the Christmas custom of Santa Clause and feel guilty that you don't believe in the customs, but you are still getting gifts.
Even worse when you're both of 'em.
1) I just decided to become an atheist. It's Christmas morning and I get out of bed to open my presents. I recall the meaning of Christmas and Christmas Guilt sinks into my stomach, and I get a :( face.
2) 12 Years old, I hop out of bed and skip on off to the tree like a faggot. I remember the Christmas custom of Santa Clause, and Christmas Guilt gets me down.