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?

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

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.