The biggest festivals first, of course, so let's start with the DELMARVA CHICKEN FESTIVAL and -- how could they -- chicken-cooking contest in the Hog Neck Arena on U.S. 50 in Easton, Maryland. Besides the chickens on display, from baby chicks to cackling hens to floats in the parade Saturday at 10, there's food for sale, a flea market, carnival, dancing and music, all with the country & chicken theme. Free admission Friday 4 to 9 and Saturday 10 to 9.

The multi-cultural, bilingual ROSEMOUNT DAYCARE CENTER has a spring festival Saturday 11 to 4. Clowns, international foods, games and music are at 2000 Rosemount Avenue NW (at the corner of Adams Mill and Klingle Roads) and it's free admission. Call 265-9885.

It's the end of spring, and the 4900 block of Just Street NE chooses to celebrate, for some inexplicable reason, the transition from warm to hot. Saturday noon to dark will be arts and crafts, music, dancing and food: free admission at Just Street (just off 49th Street between Nannie Helen Burroughs Avenue and Sheriff Road).

COLUMBIA, MARYLAND says happy birthday to itself this Sunday. Fifteen years old and running hard: There's a birthday half-marathon at 8 that morning and races, live music and an all-you-can-eat- contest noon to 10. For information, call 301/992- 5800.

Antiques to zinnias, the GEORGE MASON UNIVERSITY outdoor markets beginning this weekend offer fine arts and handicrafts for sale and live music to boot. Held the second Saturday of each month, 10 to 4, at the University in parking lot K (near the Braddock Road and Route 123 intersection). Free admission.

The third annual Northern Virginia ANTIQUE SHOW and sale s at Tyson's Westpark Hotel this Friday and Saturday noon to 9, until 6 on Sunday. Fifty exhibitors from 10 states are there for the serious antiquers. Admission $2.50 for all three days on Route 7.

For a festival with a comic flavor, try the COMIC BOOK CONVENTION at the Gaithersburg Community Center: 45 tables full of comics for show and sale, movies, and big names in the world of zap, pow & bam. Saturday 10 to 4 at 810 South Frederick Avenue. For information call 963-8804.

Manassas has a really big flea: the CRAFTS FESTIVAL at Prince William County Fairgrounds features 200 craftmen showing and selling, and a petting zoo with 50 animals to greet visitors. Friday noon to 6, Saturday and Sunday 10 to 6. Admission is $3 and parking is free.

Vienna turns itself into a garage sale Saturday 9 to 5 in a WOLF TRAP BENEFIT: the sale, expected to fill five garages, is at the model-homes section of Wesley at Balmoral, a subdivision on Kedge Drive. Go north on Lawyer's Road 1.2 miles and right on Kedge. Call 938-4200.

Here's one for the animals: VIENNA COMMUNITY CENTER'S annual summer sale benefits one woman and the 200 dogs she takes care of at Chantilly's animal shelter. For sale Saturday 11 to 7 at 120 Cherry Street there'll be appliances, books and other goodies brought by friends of the animals; donate your own or come to buy. Call 281-6313.