There are people around me who use terms, of which they do not know the meaning. I find them incredibly annoying. Yet I had a hunch that I am one of them ... and alas, I was correct. I thought that the "Twelve Days of Christmas are twelve days leading up to Christmas Day (especially given that on Boxing Day, most of my Northern English neighbours packed up Christmas to be ready for the New Year). Boy, was I ever wrong.
Wiki says: "The twelve days of Christmas in the song are the twelve days from the birth of Christ (Christmas, December 25) to the coming of the Magi (Epiphany, January 6, or the Twelfth Day). Thus, the twelve days are December 25 through January 6. (...)
Twelfth Night is defined by the Oxford English Dictionary as "the evening of the fifth of January, preceding Twelfth Day, the eve of the Epiphany, formerly the last day of the Christmas festivities and observed as a time of merrymaking."