Business is booming at the auction house sales this month, signaling the beginning of the antique season, but good buys can also be found at area antique exhibits and fairs:

* The last day of the Tailgate antique show and sale takes place today, 10 a.m.-5 p.m. on the grounds of Laws Antique Complex, 7209 Centreville Road (Route 28) in Manassas, Va. Sponsored by the Complex Antique Dealers' Association, the festival will include verbal appraisals of your undiscovered hand-carried antiques and collectibles for a $3 fee. Call 631-0590.

* Next weekend, the Antique Dealers' Association of Maryland will hold its sixth annual antiques show and sale at the White Oak Armory, Randolph Road in Silver Spring. The show will benefit Children's Hospital and takes place next Friday and Saturday, 11 a.m.-9:30 p.m.; next Sunday, noon-6 p.m. Call 854-0400.

* The Department of Recreation and Parks and the Babe Ruth Association of Falls Church will present the 12th annual Antiques Show and Sale at the Falls Church Community Center, 223 Little Falls St. in Falls Church, next Friday, 11 a.m.-9 p.m.; Saturday, 11 a.m.-8 p.m.; Sunday, noon-5 p.m. All proceeds will go to the youth activities of the Falls Church area. Admission is $2.50, with a discount of 50 cents to those with ads or newspaper clippings. Call 241-5077.

* The Eighth Antiques Show and Sale sponsored by the United Methodist Women of Concord-St. Andrew's Church will take place Wednesday-Friday at the church, corner of River and Goldsboro roads. Hours: Wednesday and Thursday, 11 a.m.-10 p.m.; Friday, 11 a.m.-6 p.m. Helen Barger will discuss American glass Thursday at 7:30 p.m. Admission to the show and sale is $2.50. LECTURES AND TOURS

* Patricia Fiske will lead a walk-through tour of "Flat-weaves and Horse Covers" from the Arthur D. Jenkins Collection Saturday, 10:30 a.m. at the Textile Museum, 2320 S St. NW. Call 667-0441. MANUSCRIPTS

* Imperial manuscriptsfrom the Mughal court of India will be exhibited at the Smithsonian's Freer Gallery of Art, Jefferson Drive and 12th Street NW beginning Friday. The 74 painted works, which will be shown through Jan. 28, 1982, are from 1560-1640, considered the most important period of Islamic art in India. The Mughal style of painting was used most often in book illustrations and in the production of miniature sculptures. The works are brilliantly colored and powdered in gold and silver. Open daily 10 a.m.-5:30 p.m. FIBER

* Out of Hand, 208 N. Lee St. in Alexandria, is showing handpainted needlepoint canvases by Ginny Diesel now through the end of the month. Diesel is showing her Jacobean and shell designs and will take commissions. Hours: Tuesday-Saturday, 10 a.m.-6 p.m. Call 549-0939.

* The Potomac Craftsmen Gallery's annual fiber show takes place now through Oct. 4 at the Potomac Craftsmen Gallery, in the Torpedo Factory, King and Union streets, Alexandria. Washington fiber artist Joan Griffin juried the show. Hours: Tuesday-Friday, 10 a.m.-4 p.m.; Saturday and Sunday, 11 a.m.-5 p.m. Call 548-0935. CERAMICS

* "American Clay 1981" opens the fall season of Meredith Contemporary Art, 805 N. Charles St. in Baltimore. The works of the 16 represented artists examine the current movement in contemporary clay by artists who use the clay body as a fine art medium for painting and sculpture. Among the participants: Don Reitz of Wisconsin, who recently showed at the Greenwood Gallery, and Maryland ceramic artists J.B. Hanson, Mary Biron, Christopher Bartlett, Harvey Sadow and Tom Supensky. The show runs until Oct. 5. Hours: Tuesday-Saturday, 10 a.m.-5 p.m., or by appointment. Call (301) 837-3575. COMBINATION SHOWS

* The Athenaeum (an affiliate of the Virginia Museum), 201 Prince St. in Alexandria, opens its fall season with ceramic mixed media sculpture by Laura Peery, drawings by Brian McCall and prints on handmade paper by Marguerite Richards. The three artists are the winners of the 11th Annual Juried Show, sponsored by the Northern Virginia Fine Arts Association. Hours: Tuesday-Saturday, 10 a.m.-4 p.m.; Sunday, 1-4 p.m. Call 548-0035. ARCHITECTURE

* Sandra Walker's watercolors of Washington and Baltimore scenes are on exhibit at the Spectrum Gallery, 3033 M St. NW. Hours: Tuesday-Saturday, 10 a.m.-5 p.m.; Fridays till 7 p.m.; Sunday, 2-5 p.m. Call 333-0954. FAIRS

* The Bel Air Festival for the Arts 1981 takes place today, 11 a.m.-5 p.m. behind Bel Air Town Hall, 39 Hickory Ave. in Bel Air, Md. Leaded glass, jewelry, wood crafts, dried and silk flower arrangements, pottery and more will be exhibited. Call 838-5473 or 838-4000. Rain date is Sept. 27.

* The sixth annual fall Mountain Heritage Arts & Crafts Festival will be held next weekend near Harpers Ferry, W. Va., off Route 340 in Jefferson County. The festival includes more than 160 craftspeople ranging from potters and jewelers to fiber artists and dulcimer makers. Bluegrass bands will play throughout the weekend. Admission: $3 for adults; $1 for kids. Hours: Friday-Sunday, 10 a.m.-6 p.m. Call (304) 725-2055 or toll-free at 800-624-0577.

* The Hardy County Heritage Weekend will take place next Saturday, 10 a.m.-5 p.m. and Sunday, noon-5 p.m. in Hardy County, W. Va. The fair will benefit the Hardy County Public Library and County Historical Restorations. Green flags mark the buildings to visit. Tickets: $5. Call (304) 538-6560. CRAFTS WORKSHOPS

* David Collins will offer a 10-week workshop in woodworking and furniture design beginning Oct. 5 at the Chevy Chase Community Center, 5601 Connecticut Ave. NW. The workshops are sponsored by the D.C. Department of Recreation and cost $35 for beginning students and $50 for advanced. Registration will take place Sept. 26, 1 p.m. at the center. Call 282-2204.

* Mary Alice Hearn will present three weaving classes starting this week in Montgomery County, near White Flint Mall. Frame loom weaving is for beginners (Sept. 28-Nov. 2), $34.50. Four-harness weaving is for students interested in learning how to operate a loom with various loom controlled patterns, (Sept. 30-Nov. 18)., $93--including loom rental; classic handwoven clothing is for weavers who want to weave a jacket using a commercial pattern, (Sept. 25-Nov.13), $95. Call 946-8358.

* The Yarn Bazaar, 714 King St. in Alexandria, begins its fall season of workshops this week. Classes include procion dye with Domine Nash ($15), knitting ($35), beginning and advanced weaving ($65 plus $35 for loom rental), spinning ($25), crochet ($35) and soft sculpture ($35).

The Yarn Bazaar carries natural, procion and acid dyes. Call 548-0408.

* Reena Kazmann will offer needlepoint Christmas gifts/decorations workshops to all ages at a location of your choice on weekday evenings or any time on weekends. Sample projects: waste canvas sachets or pincushions, book marks, plastic canvas coasters, Christmas tree ornaments and stockings, gift tags. Fee: $35 per hour, includes all supplies. Call 861-0088. Family groups welcome.

* The University of Maryland Student Union Craft Center is offering workshops and lectures now through December. Classes include glass, ceramics, textiles, calligraphy, furniture refinishing and photography. The "Starving Artist? Lecture Series" includes how to start a cooperative and custom design and finding a market. The series is free. For details call 454-4754. USEFUL KNOWLEDGE

* The Museum of African Art, 318 A St. NE, will hold an African art conservation workshop Oct. 1-Nov. 12, 6-8 p.m. with museum conservator Renee' Welfeld. Fee: $60.

* Hechinger's Do-It-Yourself Show will take place at the new Hechinger Mall, 17th and Benning Road NE next Friday, 5-9:30 p.m.; and next Saturday and Sunday, 9:30 a.m.-6 p.m. More than 40 demonstrations on home repair and remodeling will be held. Free. COLLECTIBLES

* C.G. Sloan & Co., 715 13th St. NW, will hold an estate auction beginning Thursday, 10 a.m. The sale, which runs through next Sunday, includes a Newport Queen Anne mahogany tea table valued between $30,000-40,000. Hours: Thursday-Saturday, 10 a.m.; Sunday, 11 a.m. Call 628-1468.