Original: "Simpsons Roasting on an Open Fire", also known as "The Simpsons Christmas Special", is the first full-length episode of The Simpsons to air despite originally being the eighth episode produced for season one.
Simplified: Simpsons Roasting on an Open Fire is an episode of the Simpsons from Season 1. It was the first full length episode of the Simpsons to be aired.