FRIDAY Sesame Street 20...And Still Counting (NBC, 8-9 p.m.): The Emmy-winning show so many of you grew up on - or as adults watched with your own children - is marking its 20th birthday. Hosted by Bill Cosby, the party includes guests like opera star Pl'acido Domingo, pop/blues singer Ray Charles, and some friends who were there from the beginning: the Jim Henson Muppets. Great Performances (PBS, 9-11:30 p.m.): For viewing awash in Sigmund Romberg romanticism, this New York City Opera production of ``The New Moon'' at Wolftrap should serve well. Set in 18th-century New Orleans, it includes ``Stouthearted Men,'' ``Lover Come Back to Me,'' and all the other lush, memorable numbers. SATURDAY Major league baseball (NBC, 3-6 p.m.): The network begins its 32nd year of ``Game of the Week'' broadcasts with the L.A. Dodgers at the Atlanta Braves, or the Pittsburgh Pirates at the Chicago Cubs (depending on your viewing area). TUESDAY Schoolbreak Special (CBS, 4-5 p.m.): ``A Matter of Conscience'' offers a powerful cast - Viveca Lindfors, Eli Wallach, and Tom Aldredge among others - in a troubling story about a teen-ager's love for his grandfather and the way it is shaken by some grim information about the man's past in Nazi Germany. WEDNESDAY National Geographic Special (PBS, 8-9 p.m.): Through the eyes of wildlife photographer Baron Hugo van Lawick, ``Serengeti Diary'' takes a documentary look at some the animals and people of Tanzania's Edenlike National Park.