WHERE & WHEN BLOB'S PARK is on Blob's Park Road off Route 175 at the Baltimore-Washington Parkway. SUNDAY there'll be a Sonnenwend Feier Festival celebrating the longest day of the year.

Gail's face is lit from within. And from without, with the glow of a turn on the dance floor. Short of breath, she flutters her hand to her heart. t"Oh, that Herb," she sighs as she slips into a chair at a long, long table for 20. "Watch out for him. He loves to make you dizzy."

It's Friday night at Blob's Park in Jessup, and the band is lined up and blaring out polka after polka after waltz after polka. The floor is crowded. Older couples glide around the perimeter, eyes half-shut; young couples in jeans and sundresses bob up and down. It's heavy going on the dance floor, but Herb, leaning back, smiling and twirling a new partner, is oblivious.

Herb, Herb Frederickson, teaches polka and Austrian and Bavarian dancing at the Open University, through Montgomery County Recreation Department and at Blob's Park. One Friday night a month he reserves several tables and invites his students, former students and prospective students to join him there for an evening of practicing what he has preached.

"Don't just dance with your spouse or your date," he tells a newcomer as he shepherds her to the dance floor. "That's so dull. Dance with everybody. That's what's so much fun." He leans back and twirls his partner. "Do you like being dizzy?" he asks, answering his own question: "I do, but it's an acquired taste." Watch out for Herb.

Watch out for Paul Jones, too. Paul Jones is a mix-and-mingle dance that comes up three or four times a night. Everybody grabs a partner and polkas. When the whistle blows, women form a circle on the inside looking out; men form a circle on the outside looking in. Round and round they go -- women heading clockwise, men counter. When the whistle blows again, it's grab as grab can -- only somehow the numbers are off, and there aren't enough partners to go around. With a sharp lack of tact, an announcer calls out, "Leftovers, into the center of the ring."

Blob's Park, a cavernous beer hall that seats and dances close to a thousand, is not a place for heavy conversation. Just about everyone who comes -- grandmothers and grandfathers, young parents on a night out, singles from Washington and Baltimore and a whole lot of other people -- comes to dance, and to watch the dancing. But the floor is so packed and the action so intense that styles don't matter -- you're lost in the shuffle of polka-ing humanity.

As for clothes, anything goes as long as it keeps you cool. Some of the more serious folk dancers wear traditional polka costumes: leather knickers or leiderhosen for the men; bright, very full, low-cut peasant dresses for the women.

As the night wears on -- the band plays from 9 to 1 on Friday and Saturday nights, 5 to 10 on Sundays -- pitchers of beer are drained, knockwursts with hot bacon-flavored potato salad are downed. Big bowls of large, hard pretzels disappear. And the polkas go on and on.

The best way to visit Blob's Park is to bring your own crowd. Tables are for 20, but you can reserve the number of seats you want. Admission is $2 on Friday or Saturday night; $1 on Sunday.

Another way is to attend one of their festivals, with dancing and entertainment inside in the beer hall and outside on a pavilion, German dance and music groups, and concessionaires selling sausage, beer, German sheet cake and gifts. The fesitivals attract some 2,000 people on a bright, sunny day.