In this episode, set in 1976, Kelso shows up at Donna's party, which he mistook as toga-themed. He recites the famous chant from the film, "Animal House", which was not released until 1978. Of course, "Animal House" didn't invent the chant, it simply reflected what had previously been chanted at toga parties for years.
Donna was having a party at her house because her parents weren't home. Donna had a little sister at this point, but later in the series, she was an only child.