September 19, 1986

AS of Sunday, it's officially autumn, which means that the peak season for New York street fairs and festivals is drawing rapidly to a close. It's almost as if this weekend had been planned as a grand finale, for there will be no less than eight neighborhood celebrations tomorrow and Sunday - not only in Manhattan, but in Queens, Brooklyn and Staten Island as well. The most spectacular of these is undoubtedly the Third Avenue Festival, which will take place on Sunday from 11 A.M. to 5:30 P.M. between 68th and 90th Streets. Hundreds of thousands of fairgoers are expected to wander through this pleasant Upper East Side neighborhood, shopping, eating, playing games and listening to music. The Third Avenue Festival also boasts its own 80-piece orchestra. The New York Pops, under the direction of Skitch Henderson, will play between 72d and 73d Streets around 2 P.M.; in addition, there will be continuous entertainment on the site from 11:30 A.M. to 3 P.M. The ''Celebrity Corner,'' in front of Jim McMullen's restaurant, between 76th and 77th Streets, will allow passers-by to meet entertainers, television personalities and sports figures from the New York area.

International Mikhail S. Gorbachev rebuffed personnel assurances from President Reagan that a detained American journalist was not an American spy. The Soviet leader, in his first public comments on the case, said the journalist, Nicholas S. Daniloff, was ''a spy caught red-handed'' whose detention was being used by Washington to ''reap a harvest of hatred'' against Moscow. [ Page A1, Column 6. ] 25 members of the Soviet mission to the United Nations who must leave by Oct. 1 are senior intelligence officers, Reagan Administration officials said. They also said the ouster of the 25 would severely hurt what they called the Kremlin's most important espionage outpost in this country. [ A1:5. ]

A year ago the five major industrial powers forged a historic compact to drive down the value of the dollar in hopes of easing a crushing trade imbalance. The drop in the dollar has been stunning. Since the meeting of the five nations last Sept. 22 at the Plaza Hotel in New York, the dollar has plunged 55 percent in value against the yen and 40 percent against the West German mark. But the broader objectives of correcting trade flows and reinforcing international cooperation have proved stubbornly elusive. The United States faces a trade deficit approaching $200 billion this year -up sharply from $148.5 billion in 1985 -along with a rising clamor for protectionism and increasing strife with its leading trading partners over their reluctance to spur economic growth.

Friday STAMP FESTIVAL '86 Stamp collectors and enthusiasts gather this weekend for Stamp Festival '86, a three-day showcase at Madison Square Garden. Sponsored by the American Stamp Dealers Association, the exposition offers an array of lectures, seminars and exhibits, with auctions at noon tomorrow and Sunday featuring more than 2,000 lots. The festival, which will be in the Garden's Exposition Rotunda, runs from 11 A.M. to 7 P.M. today, 10 A.M. to 6 P.M. tomorrow and 11 A.M. to 5 P.M. on Sunday. Daily admission is $3, with children under 14 admitted free if accompanied by an adult. Information: 564-4400. Saturday BLACK ROOTS IN NEW YORK Anyone with an interest in black culture or regional black history might want to stop in at ''African Roots in New York Soil: Black Culture in Colonial Westchester'' this weekend. The two-day program at Philipse Manor Hall in Yonkers begins Saturday night at 8 with a keynote address by Dr. Thomas J. Davis, whose lecture is entitled ''Black Culture in Colonial New York.'' Sunday's events begin at noon with a church service by the Rev. Darryl George of Yonkers Messiah Baptist Church, followed at 1 P.M. by the gospel soloist Tulivu-Donna Cumberbatch. The Olatunji Dance Troupe performs at 2 P.M., and at 3:45 there will be a dramatic piece based on an 18th-century slave narrative, delivered by Charles Dumas. At 4:30 P.M. comes the finale, a ''call and response'' gospel performance by the Brooks Family of Rockland County.

Today is the day when the stock market, still spooked by last week's dramatic drop in prices, just might rebound strongly. But it is equally likely that today's market, in its closing minutes, could produce another wrenching fall that would rattle the investing public even more. On the third hand, maybe nothing much will happen. Such is the uncertainty and the suspense in today's ''triple witching hour,'' a quarterly phenomenon that has become synonymous with wild, cascading swings in stock prices. The witching hours occur on the third Friday of the final month of each quarter, when three sophisticated sorts of investments - contracts on stock index futures, stock index options and individual stock options - all expire.