Not a freelancer (yet?), but I know that most organization’s budgets end on December 31st, which means a promotion mailer that stands out in December can translate into an order paid by budget monies which need to be spent by year-end. A good promotion during the holidays could also keep you top of mind in January when they’re deciding on where to spend over the next six months.
Very clever promo, btw. Definitely gained an admiring smile from me.