I reckon they got the idea from http://www.24ways.org http://www.24ways.org, which in turn is inspired by the advent calendar (http://en.wikipedia.org/wiki/Adve

I reckon they got the idea from http://www.24ways.org http://www.24ways.org, which in turn is inspired by the advent calendar (http://en.wikipedia.org/wiki/Advent_calendar http://en.wikipedia.org/wiki/Advent_calendar). "The traditional [advent] calendar consists of two pieces of card stock on top of each other. Twenty-four doors are cut out in the top layer, with a number ranging from one to twenty-four on each. Beginning on the first day of December, one door is opened each day [containing a chocolate and/or showing an illustration related to Christmas], counting down the days remaining until Christmas Eve, from one to twenty-four where the 24th door often holds an extra surprise like an extra large piece of chocolate."