Considering the first prize is from the first episode of the season (Season Premiere) and the second prize is from the Treehouse of Horror episode of the season, I wonder if the third, fourth and fifth prizes will be from the Christmas/Winter (if there is one this season) episode, Valentine's/St. Pats/Easter episode (if there is one this season) and the last episode of the season (Season Finale).
I think it will be highly likely with the fifth prize being from the last episode but for the third and fourth prizes it could be what I mentioned above about them being from seasonal episodes or it could just be 2 random episodes from the season.