From About.com
Perhaps it's been going on my entire life, but I first noticed the
phenomenon a few years ago. Starting on December 13 or 14, depending on
how mathematically/calendrically challenged the particular blogger or
business is, the countdown to Christmas begins: "On the First Day of
Christmas [we put this on sale | I recapped the top stories of January | etc.]."
Except, of course, that December 13 is the Feast of Saint Lucy and December 14 is the Feast of Saint John of the Cross, and neither day is the "First Day of Christmas," because they both fall in Advent.