Ferris Bueller's Day Off follows a linear structure, and it does so while intertwining three distinct stories: Ferris, Cameron, and Sloane's Chicago adventure (the A story); Rooney's hunt for Ferris (the B story); and Jeanie's quest for justice (the C story). The film may cut back and forth between the three tales—and frankly, how much ground Ferris and his pals manage to cover may be highly suspect—but the movie's narrative keeps chugging along in a clear, chronological order.
The movie also uses an embedded narrator: Ferris. He doesn't just remark on what's happening; he participates in the events, too. It's like if Harry Caray not only commentated on Cubs games, but also played shortstop while doing so.
On top of that, Ferris also breaks the fourth wall at regular intervals, delivering his insights and opinions by addressing the audience directly. Here's why: First, it engages the audience. For example, Ferris doesn't just rattle off tips for faking sick, he explains his techniques directly to you—yes, you—using on-screen bullet points. Second, it's a means of unleashing insights about other characters (ahem, Cameron) that the audience wouldn't be able to get otherwise.
So much of understanding the impact of Ferris, Cameron, and Sloane's day off in the narrative present rests on knowing the backstory that only Ferris's frequent, fourth-wall-breaking monologues can dispense.