* It wouldn't be fall without plenty of fairs and festivals. And as if last weekend's celebrations weren't enough, here's a whole new crop to choose from:

* Decatur House Museum hosts its American Crafts Fair of 19th-century urban crafts, including basketry, pewter, caning, pottery, scrimshaw, quilting and silhouette- cutting this Friday, Saturday and Sunday from 10 to 5 at 748 Jackson Place NW. $3, $1.50 for children under 12. For more information, call 673-4030.

* There'll be live music, square dancing, apple butter making, blacksmithing, plus plenty of crafts and food for sale at the Old- Fashioned Fall Festival at the Merrifield Garden Center in Merrifield, Virginia, this Sunday from 9 to 5. The center is at 8132 Lee Highway; admission is free. For more information, call 560-6222.

* The Tenth Annual National Craft Fair is this Friday, Saturday and Sunday from 10 to 6 at the Montgomery County Fairgrounds. It features 400 craft booths, entertainment and ethnic and regional foods for sale. $4, free for children under 12. Call 301/258-8870 for directions and information.

* The Apple Harvest Festival in Sperryville, Virginia, is this Saturday and Sunday from 10 to 6, with horse-and-buggy rides, crafts, food, beer and apples galore for sale, as well as live music and an exhibit, "Artists of the Blue Ridge," by more than 30 regional artists. Admission is free. Call 703/987-8521 or 703/987-8761 for directions and information.

* Berrywine Plantations Winecellars holds its Oktober Wine and Foliage Festival this Saturday from 10 to 6 and Sunday from noon to 6 at 13601 Glisans Mill Road in Mt. Airy, Maryland. There'll be live entertainment, crafts, food and wine for sale and helicopter rides ($10.50 for 5 minutes). Admission is free, parking is $2 and a tour of the winery is $2. Call 301/662-8687 for directions and information.