So just what are the 12 days of Christmas, and who decided what 12 days they are? Are they the 12 days leading up to Christmas, or the days following Christmas? If you guessed "B", you would be correct. And if you did, you are likely from a background where the Christian liturgical calendar is observed in detail. The 12 days begin on Christmas Day and lead up to Epiphany, the day when the coming of the magi (wise men) is observed.