I think that "Christmas Calendar" is a bit of a misnomer. To be precise, this is a variation on the tradition of the "Advent Calendar". Advent is the time of preparation for Christmas. Clearly the authors of the Calendar understand this, based on the graphic under day "1" :)
Edit: on second thought, i guess you've meant "Machine Learning Christmas" Calendar. That's fine. It just bothers me a bit that in the US people forget about Advent.