NBC's archetypal comedy program "Saturday Night Live" should have died in 1985, a decade after a volcanic debut that launched stars such as John Belushi, Dan Aykroyd, Chevy Chase and Gilda Radner. And yet a half-century later, producer Lorne Michaels and his merry maniacs are still at it. What gives?

The sketch comedy show is celebrating 50 seasons with two documentaries and an upcoming prime-time special that reflect on its standing as an American institution.

A treat for everyone who saw those early years, “Saturday Night” explains why some weeks are good and others aren’t.