I just realized that "Sue Ellen Vegges Out" takes place around May 5th, as, during the beginning of the episode, a Cinco de Mayo themed lunch is served.
Yet I remember that one episode (sorry, can't remember exactly which one at the moment) from one of the earlier seasons, where Arthur walks up to his mom who is working at their house computer...on the desk is a calendar that reads May 5 (which, IIRC, flips through several days in May).
It seems like the show's been going on so long, we're starting to see more "overlapping of dates". Plus there's that whole thing about "Sue Ellen Chickens Out" taking place in the fall despite her debut taking place in a winter episode.