WORTH NOTING ON TV

* SATURDAY

Saturday Night Live (NBC, 11:30 p.m.-1 a.m. EDT): NBC promises a revitalized ''SNL,'' but will viewers return for its 21st year? The season premiere boasts a revamped cast, Mariel Hemingway as guest host, and the artist formerly known as Prince as musical guest.

* SUNDAY

Signs of Hope (ABC, 12:30-1:30 p.m. EDT): This first of four new ''Vision & Values'' specials celebrates ordinary people who are making a difference in developing countries. A production of the National Council of Churches.

The American Promise (PBS, 8-9 p.m.): Graffiti offenders in Philadelphia have channeled their talents into 1,200 outdoor murals, which have brightened neighborhoods and reinvigorated communities. In Chicago, five African-American women stand as collateral for one another to secure loans for their small businesses. These stories and others in this three-part series enlighten nine aspects of grass-roots democracy: freedom, responsibility, participation, hard choices, information, deliberation, opportunity , leverage, and common ground.

Parts 2 and 3 of the series air Monday and Tuesday.

The Lincoln Assassination (The History Channel, 9-11 p.m. EDT): Journalist Roger Mudd hosts and actor Tom Berenger narrates this in-depth documentary, which was produced in collaboration with American Heritage magazine.

* MONDAY

Nightly Business Report (PBS, broadcast time varies): Featured this week are interviews with prominent CEOs on the ''new American leadership.'' Harold McGraw III of McGraw-Hill Inc. appears tonight, followed by Edwin Artzt of the Procter & Gamble Company, Charlotte Beers of Ogilvy & Mather Worldwide, Wolfgang Schmitt of Rubbermaid Inc., and Alfred Zeien of the Gillette Company.

The Tonight Show With Jay Leno (NBC, 11:35 p.m.-12:35 a.m. EDT): Political activist Harry Wu (his recent detainment in China upset China-US relations) and singer Bette Midler are scheduled guests.

Please check local listings for these programs.