Tradition, cultural heritage and bringing together families which are spread over the world.
For children, it's the excitement, anticipation and magic of the nativity and the story of St Nicholas. Of showing love with the giving of (but mostly receiving) presents. Making a fairy story seem real (as fairy tales ever will) for them. For adults, it's the build up to a special celebration where families get together and over-indulge whilst re-establishing family bonds and catching up in a convivial atmosphere.
It's the one date in the calendar which needs no reminding.