I'd tell them to spend one or two weeks Googling it. There's so much to say that I wouldn't even want to begin.
How would you explain the Christmas Tradition, normally celebrated in the USA to someone who had absolutely no idea what it was all about?
They could do worse than reading this well know letter:
Oh my that would be very hard. No two families do things the same way or in the same order.
For instance when to put that tree up?
Real or fake one?
Ham or turkey?
Dressing or stuffing?
Stockings hung or not?
It's merely an excuse to have parties during December.