It finally occured to me. It's the hope.

After the drudgery of winter, our hopes of warmth and light are being fulfilled.

For me, this is analogous to God's promise that things will be okay. They'll get better, even, if we just keep faith. The worst possible thing happened: God died. But then He rose. I think this might have been the (unconscious?) rationale behind setting Christmas at the beginning of the winter. At Christmas the promise is made: at Easter it is fulfilled.