I think the props department made a mistake, but just for the logic of the show: there are people, like myself who have calendars from previous years hanging around. Maybe the girl loved the calendar so much that she wanted to keep it and ignored the wrong weekdays (lame, I know).
What I would love to see on the show? Another 2010 calendar or poster and Dean and Sam making fun of it! Someone should point that out on the next convention!