In The Avengers, Tony Stark debuts his latest and greatest armor: the Mark VII.
While it is not entirely conclusive, people in New York are wearing light dress during the Avengers, so we can reasonably assume that the movie takes place somewhere between Spring to early Fall.
Iron Man 3 picks up an undisclosed amount of time later. Tony is wracked with a sort of PTSD and in his sleepless nights, has upped his armor collection all the way up to Mark XLII - that's 35 new armors.
Christmas takes place over the course of the film - but of what year?