SATURDAY SCIENCE SATURDAYS: "BIOLOGY DAY," with hands-on fun, science demos, interactive exhibits and more, at Science Museum of Virginia. 864-1400. SUNDAY AND FRIDAY WOLF TRAP OPERA "Candide" today, "Alcina" Friday at Wolf Trap, Vienna. (877) 965-3872. THURSDAY-SATURDAY KINGS FEST Christian pop performers at Kings Dominion, Doswell. 876-5561. THURSDAY-SATURDAY NATIONAL SYMPHONY ORCHESTRA Different programs each night at Wolf Trap, Vienna. (877) 965-3872. FRIDAY CHRIS ROCK Superstar comedian at Landmark Theater. 646-4213. ALSO FRIDAY ANI DIFRANCO Alt-folk singer at The National. 612-1900.