What does Christmas mean? The celebration of the birth of Jesus, versus the commercialisation of today? Father Christmas? Christmas Fact and the accepted fiction?
Explaining Christmas to a baby is simple; you don't have to. However, as a child grows up, there is a lot about the world that needs to be explained; Christmas is a good example of this, as it's a fusion of tradition, religion, commerce, status, location and more. A proper mix.
Read Dawkins and you'll be wary of indoctrination; but, in a way, that's missing the point; Christmas only exists in a widely accepted fashion due to it's religious roots.
And then there's Father Christmas; how do you explain him? If you accept his existence as a necessity for children, then how do you explain his inevitable unveiling?
It was easier when you were a kid.