John Cage, who's 70 this year, will preside over big doings this Saturday at the Pension Building, where his music and words will fill a Cage celebration from seven to midnight. Then, on Sunday at 3, the grand old man of the avant-garde will read from his latest book, "Composition in Retrospect," at the Washington Project for the Arts at Seventh and D streets NW.

Saturday's program, another installment in the Ninth Street 1982 Festival, will feature early Cage works such as the Nocturne for Violin and Piano of 1947, as well as the American premiere of a 1978 piece, "Etudes Borreales for Piano." For a work by composer Earle Brown and the late sculptor Alexander Calder, "Calder Piece, Chef d'Orchestre," a free-standing mobile will somehow conduct four percussionists, who will move among 100 percussion instruments and attack the sculpture with mallets.

Graced by readings from Cage himself, the evening will end with his "Concert for Piano and Orchestra with Aria," with David Tutor, Cage's foremost interpreter at the keyboard.

All the Cage is divided into three parts, and so is the Pension Building. The audience will be expected to move from place to place -- to place -- for the full effect. Debra Hanzlik of the Washington Performing Arts Society, which is co-sponsoring the event with District Curators Inc., advises, "Bring a pillow or a sleeping bag if you want and lie out on the carpet." She also promises refreshments throughout, though they're not included in the $8.50 ticket price. Call 393-3759. Tickets for Sunday's reading, available at the door, are $5.