The celebrations this weekend fall into several categories. There are the festivals of the season, as well as art shows and craft exhibits and sales; and apparently the weather is so invigorating that it has stirred families and organizations to clear out basements and attics, making for plenty of flea markets, bazaars and yard sales. CELEBRATING THE SEASON

FRIENDS OF TREGARON hold an Octoberfest block party that celebrates that twenty-acre estate and its nearby parklands. Sunday, 2 to 4, on Cortland Place NW, near the corner of Klingle. Bluegrass and jazz music, pumpkins and white elephants for sale. Call 797-8455.

The German farmhouse SCHIFFERSTADT has its Octoberfest with German music, crafts and house tours 10 to 5 Saturday and Sunday. Free admission at 1110 Rosemont Avenue in Frederick. Call 301/663-1611.

SCHAEFER'S INNSTEAD FARM holds its second annual Pumpkin Festival Saturday and Sunday with country/bluegrass music, hayrides and pony rides 11 to 4, and pumpkin-picking 10 to 5 at 18020 Edwards Ferry Road in Poolesville. Call 972-8091.

CIDER MILL holds a Halloween Festival Saturday and Sunday. There's story-telling and a costume contest for kids, and food for sale, including Indian corn, gourds, pies and breads. Tours of the mill and free samples of the cider. Near Ellicott city: north on I-95 to the Route 100 exit, left at end of ramp and continue two miles; left on Montgomery Road, then right on Landing Road and continue two miles. Free. Call 301/788-9595.

At the WESTMORELAND CONGREGATIONAL CHURCH, a pumpkin and bake sale benefits the scholarship fund of Westmoreland children's center. Saturday 10 to noon, at 1 Westmoreland Circle (on Massachusetts Avenue). Call 244-4291.

Georgetown Day School holds a COUNTRY MARKET DAY Saturday, 10:30 to 3:30, at 4530 MacArthur Boulevard NW.

The National Wildlife Federation sponsors FALL DISCOVERY DAY Saturday with nature classes featuring live birds of prey, as well as demonstrations of rail-splitting, flycasting and taxidermy. Pottery, rug-hooking and candlemaking are among crafts demonstrated and for sale. At Laurel Ridge Conservation Education Center at 8925 Leesburg Pike in Vienna, two miles west of Tysons Corner. Call 797-6850.

Northern Virginia Community College has its OCTOBERFEST at the Alexandria campus, Saturday at 8 p.m. Call 931-6417.

A HARVEST FESTIVAL at the National Presbyterian School, 4121 Nebraska Avenue NW, is 10 to 2 Saturday.

In Pomfret, Maryland, the Afro-American Heritage Society presents its sixth annual HERITAGE DAYS celebration, Saturday and Sunday at McDonough High School. Exhibits, music, food and historical displays go on noon to 6 both days. Donation of $1 requested.

And one more: The Washington Sheraton opens its doors for a WASHINGTON EXTRAVAGANZA to benefit the American Cancer Society. Two rock bands, cash bar 9 p.m. to 1 a.m. Admission $5; proper dress required at 2660 Woodley Road and Connecticut Avenue NW. Call 864-8803 or 699- 5499.

ART AUCTIONS AND SHOWS

An auction to benefit the Muscular Dystrophy Association has a twist: the Metropolitan Dry Cleaners Association has a VACATION AUCTION where it auctions off paid vacations, from New Jersey trips to ones in Acapulco. Friday, 7:30, at the Holiday Inn of Bethesda, 8120 Wisconsin Avenue. Donation $2. Call 466-7450.

Old Town's Carlyle House has an ANTIQUE PRINTS SALE with art from the 17th, 18th and early 19th centuries Friday and Saturday 10 to 5, Sunday noon to 5. Admission free at 121 North Fairfax Street. Call 549-2997.

The Japanese American Citizens' League holds AKI- NO-ICHI, a fall Japanese bazaar, Saturday at Walter Johnson High School. Japanese foods, arts and crafts, 11 to 4, at 6400 Rock Spring Drive (off Old Georgetown Road.) Call 296-4484.

An ART AUCTION in Bethesda Friday at 7:30 benefits the NIH Daycare programs; tickets are $2.50 and art includes sculptures, paintings and prints by Picasso, Miro, O'Keefe, Nieman, Rockwell, and others. At Stone Ridge School, 9101 Rockville Pike. Call 496-5144.

White Oak Armory has a FALL CRAFT SHOW Saturday and Sunday, 11 to 5, sponsored by the Olney chapter of Women's American Organization for Rehabilitation through Training (ORT). Arts and crafts by 50 artists are shown and sold at U.S. 29 and East Randolph Road in Silver Spring. Call 924-3466. And the Fairfax chapter of ORT holds its fourth ART AUCTION Saturday at the Springfield Hilton, 6550 Loisdale Road, selling oils, graphics, enamels and more. Call 250-6023.

The annual Cherry Hill NEEDLEWORK SHOW is Saturday at Falls Church Community Center, 223 Little Falls Street. See and buy old and new handmade quilts, needlepoint, Christmas ornaments, embroidery and laces. Cost $1.50. Call 241-5003. FLEA MARKETS & BAZAARS

Junk that might be gems for others is at a number of flea markets and bazaars:

A USED COSTUME SALE is sponsored by the Catholic University Drama Department, 10 to 3 Saturday in the lobby of the university's Hartke Theater.

ST. PAUL UNITED METHODIST CHURCH holds its annual bazaar Saturday 10 to 4, featuring a Christmas Corner as well as Halloween and Thanksgiving crafts, and food for sale. On East- West Highway and Grubb Road in Chevy Chase.

ST. THOMAS APOSTLE CHURCH has a bazaar Saturday, 9 to 4, at 2665 Woodley Road NW.

ST. GEORGE'S EPISCOPAL CHURCH sponsors a bazaar Friday and Saturday, 11 to 6, at 160 U Street NW. Call 387-6421.

CHERRY HILL FARM has a little of everything Saturday 3 to 8: an auction, a flea market and music -- all at Gallahan Road in Piscataway, Maryland. Call 292-2500.

THE UNIVERSITY FELLOWSHIP CLUB sponsors its third annual bazaar Friday and Saturday in College Park with a flea market and baked goods for sale, at 3515 Campus Drive. Hours are 10 to 3. Call 422-7970.

THE SOROPTIMIST CLUB sponsors at flea market, crafts and bake sale Saturday at Glebe Elementary School, 1750 North Glebe Road in Arlington, 9 to 3. Call 697-6902.

In Fairfax, the barbershop's JUBIL-AIRES chorus sponsors a flea market at the Fairview Shopping Center (one mile east of the courthouse on Route 236) Saturday. They'll be singing as well, Saturday 9 to 4. Call 280-1531.

And the NATIONAL COUNCIL OF JEWISH WOMEN fill the Yorktowne Shopping Center, at U.S. 50 and Gallows Road, with a flea market Sunday 2 to 5. The Women's Association of the NATIONAL PRESBYTERIAN CHURCH holds its bazaar Saturday, 10 to 3, at 4101 Nebraska Avenue NW. Antiques, a hat boutique and crafts; and appraisals of collectibles, $5 for three items. 667-5109.

The WASHINGTON ETHICAL SOCIETY SCHOOL sponsors a multi-family bazaar at 7750 16th Street NW, Saturday 10 to 4.

St. Gertrude's Guild sponsors a bazaar Saturday at ANNUNCIATION CHURCH parish hall, 3810 Massachusetts Avenue NW, 11 to 4.

The ZEN BUDDHIST CENTER sells trash and treasures Saturday and Sunday, 9:30 to 4, at 500 Butternut Street in Takoma Park.