DISCOUNTS START TOMORROW on all major airlines, averaging $20 (one way) below regular max-saver fares until mid-December. Texas Air's Continental announced discounts first, and other airlines followed with similar cuts. Restrictions vary. Some airlines require reservations 14 to 30 days ahead, and refunds range from zero to 50 percent; all specify limited seats per flight. GASOLINE PRICES AT THE PUMP averaged $1.12 per gallon in the US over Labor Day weekend 1987, according to the American Automobile Association. That's an increase of 10 cents a gallon above July 4 weekend prices and 20 cents above prices on Labor Day weekend in 1986. NAVAJO INDIANS WILL GATHER in Window Rock, Ariz., to celebrate the 41st annual Navajo Nation Fair and Pow Wow, Sept. 9-13. Events include traditional singing and dancing, a rodeo, and an art competition. For details call Brenda Eastin, Arizona Office of Tourism, (602) 255-3618. MINNESOTA FALL FOLIAGE GETAWAYSare listed in the Minnesota Explorer, a 12-page newspaper, available on request from the Minnesota Travel Information Center, 375 Jackson St., 250 Skyway Level, St. Paul, MN 55101; telephone (800) 228-5400. VISITORS TO ORLANDO, FLA., can get discounts on hotels, restaurants, and entertainment with the ``Fall Celebration'' coupon book, available for $1 from the Orlando/Orange County Convention & Visitors Bureau, PO Box 593788, Orlando, FL 32859.