SATURDAY PBS: Live From the Met (2 p.m., 8 p.m.; Wed, 8:30 p.m.) Centennial star-studded celebration of the Metropolitan Opera, featuring just about every important voice of today.
ABC: 14th Annual New York Marathon (10:30 a.m.-2 p.m.) Live coverage of this annual 26.2-mile run through New York.
SHOWTIME: A Case of Libel (2:30 p.m. and 8 p.m.; also Oct. 28 and dates to be specified in November) Pay cable version of Broadway play concerning the Quentin Reynolds lawsuit against Westbrook Pegler, starring Dan Travanti and Ed Asner.
PBS: Nature (8-9 p.m.) Acacia trees in Tanzania's Serengeti Plain are featured in this segment of one of TV's best science shows.
PBS: The Life of Verdi (8-10 p.m.)
Part I of six-part dramatization of life of the Italian composer.
NBC: He Makes Me Feel Like Dancing (4-5 p.m.) Special treat for young people about young New York ballet students, with Jacques d'Amboise.
NBC: Bay City Blues (10-11 p.m.)
The creators of ''Hill Street Blues'' are premiering a new series about a minor-league baseball team. Recognizable Hill Street touch, but it may leave non-baseball fans a bit puzzled.
Please check local listings, or with local stations, for all programs, especially ones on PBS.