WORTH NOTING ON TV

* SATURDAY

20 Years of Comedy

on HBO (HBO, 10:30 p.m.-midnight): For 20 years a range of comedy material has been aired on HBO - from live stand-up acts once not seen on TV, to sitcoms and other series. This special recaps the rich succession of talent to be found during those two comedy decades on HBO. The retrospective makes for an interesting overview of cable TV culture, although the clips from several acts contain offensively explicit language that some viewers may wish to avoid.

Appropriately enough, the host is George Carlin - star of his own series on Fox - who has done more HBO comedy specials than any other comedian. He introduces clips of early appearances from people like Jerry Seinfeld, Tim Allen, Brett Butler, Jim Carrey, Ellen DeGeneres, Paul Reiser, and Roseanne. Glimpses are offered of shows like the ''Comic Relief'' benefits. Seinfeld, Garry Shandling, Billy Crystal, Whoopi Goldberg, and others are interviewed.

* MONDAY

Dead Ahead: The Grateful Dead in Concert (PBS, 8-9:20 p.m.): When they appeared in San Francisco many years back, the Grateful Dead didn't even have to use the name of the band in their ads, which read: ''They're not the best at what they do. They're the only ones who do what they do.'' Deadheads, after all, are probably the most committed of all rocks fans and range from toddlers and grade school dropouts to senators and Nobel laureates.

They're currently marking their 30th anniversary, but this special offers highlights from a 15th anniversary show at New York's Radio City Music Hall in 1980. Deadheads lined up days in advance to buy tickets for that concert, which proved an important milestone in the band's career.

The band members appearing in this show are Jerry Garcia (guitar, vocals), Bob Weir (guitar, vocals), Phil Lesh (bass, vocals), Bill Kreutzmann (drums), Mickey Hart (drums, percussion) and the late Brent Mydland (keyboards, vocals).

Please check local listings for these programs.