The phrase "TGIF" was first used on the episode where Potsie stays over for the night while Richie's parents are away, and the three women with the broken down car stay the night also. They are sitting around the living room in a general good mood and Richie suddenly says, "TGIF," out of nowhere. They look at him confused, until he says, "Thank God it's Friday."

It was a very dumb and hilarious phrase that seemed to grow in popularity after that. A few years later in 1978 a disco party movie called Thank God It's Friday came out.

I was watching the episode with my parents when it first appeared and I remember them laughing at the line. I guess because it was such a dumb thing to say.