What song Mr.Halloway hear ? Why is it unusual to hear it in October?

It's October and Mr. Halloway hears a man humming Christmas song. What you need to understand is that this novel is filled with Halloween. Christmas is completely out of place in this setting! Even more incongruous is the fact that the "hummer" is actually one of the owners of the evil carnival. Enter .......... Mr. Dark. But you're not supposed to know that yet, so keep reading!