There are any number of things that can trigger a memory of childhood Christmas. It might be the pine smell of the Christmas tree that you remember as a child. The way the closets smelled at your grandparents house. The sweet smell of pipe smoke from a blend your dad used.
For me, it's sugar cookies baking. That smell takes me back, I can see the house and the decorations, remember being six years old and just about split in half with anticipation. We made some this week and in the midst of all that is happening in 2015, they still worked their magic.