They never wore beaver after Christmas. They went into pastels and velvets until Easter gave assent to straw and flowers. Back then a lady needed at least 15 hats for the year and at least four to get through the day: a lunch hat, a tea hat, a cocktail hat and an evening hat. The differences were subtle but crucial. You didn't go to the butcher's without the right hat. To be bare-headed was akin to being barefoot.

Then the winds of change blew hats out of the fashion picture. Some blamed hairdressers and the bouffant, others blamed the Pope for giving his blessing to hatless worship. But whatever the cause, hats were out and millinery shops disappeared like dinosaurs in the Ice Age.

Now, after over two decades of untrammelled tresses, designers dictate that hats are back. Retro fashion, they call it. The vogue is for saucy little chapeaux with sequins, flowers and feathers - like the old days.

Sonia Sheftell knows all about sequins, flowers and feathers: She's got drawers full of them.Only they're not retro anything: They're the originals - the flowered pill-boxes she made for Mamie Eisenhower, the straws Alice Roosevelt Longworth ordered faithfully until her 91st birthday. Sonia Sheftell is a holdover, a milliner who has survived the Ice Age. She credits her survival to her carriage trade, the ambassadors' wives, the cabinet wives, the givers of garden parties and regal receptions.

"Darling, my customers never stopped wearing hats," Mrs. Sheftell states in the throaty Russian accent that clings to her speech even after 57 years in this country. "They had millions, darling. Money didn't mean a thing to them. It was class all over."

Mrs. Sheftell is watching Connecticut Avenue roll by from her brocade throne in the Regis Room (1613 Connecticut Avenue NW, 667-0564), the millinery shop she opened September 17, 1945. With her thick gray hair sleeked back into a chignon, trim in pearls and heels, she has an elegance that hasn't faded with time. Unfortunately, she feels that her surroundings haven't fared as well. "It was a different avenue then. There used to be Rolls-Royces and chauffeurs on the Avenue, now there are delivery trucks. There were 21 millineries on Connecticut Avenue when I opened up. Now I'm the only one left."

Chauffeurs eased their society ladies along Connecticut Avenue from shop to shop and then to the Sulgrave Club or the Washington Club for lunch. You can almost hear the click of high heels and the trill of moneyed voices as Mrs. Sheftell recalls the ladies trying on hats and gossiping about last night's dinner party.

"Oh darling, she was a honey," Mrs. Sheftell says pointing to a picture of Alice Longworth Roosevelt. "She spent hours in my place. She'd come in every spring and order a five-inch-brim Milan straw in brown, navy and black." Then there was Daisy Harriman, who always wore her spring hats trimmed with daisies. And there was the time Lynn Fontanne played the old Belasco Theater on 9th Street NW, and had Mrs. Sheftell make her a pillbox from French voile. "She said I was an artist," Mrs. Sheftell recalls. "Oh, the hats I used to make for them all, darling, it breaks my heart to think about it."

She glides under the white lattice arch that separates her showroom, now festooned with flowered hats, to her workroom, and sadly looks around at the boxes of artificial posies and seed pearls. "Now it's all junk. Italy stopped making Borsalinos. The Czechs used to make the most beautiful flowers, they were like fresh. Now they come from Japan. You can't get French velvet or satin. There's no more real chiffon like we used to make hats from. But if there was, who'd wear them? They're all bald old ladies now who don't go out. And their children and grandchildren lead different lives. They don't have the servants, the homes. It's a different world. . . I sell mostly to shopgirls now."

Israel Bachrach remembers the Easter Parades on Connecticut Avenue. There were no marching bands, no majorettes, just local folk showing off their Easter Bonnets, just like in the Irving Berlin song. "It would start at churchtime Easter morning. People - especially women - would try to outdo each other. Oh, it was a big deal with prizes for the best-looking Easter outfit and pictures in the newspapers. But now they all drive to church. There hasn't been an Easter Parade in 25 or 30 years." Bachrach shakes his head. "They say hats are back, but they can't wave a magic wand and bring back an industry."

Bachrach, a gentle little white-haired man, is also a survivor of that industry. He's been in the hat business for 70 of his 79 years, starting out in his Polish immigrant father's Ninth Street store when he was a schoolboy. Thousands of hats have passed through the hands that nimbly peel an orange in the afternoon quiet. Bachrach is upstairs eating his lunch. Below him on the ground floor of Bachrach's Millinery (733 11th Street NW, 628-4194), his wife, daughter, sister-in law and two trusty old helpers scurry about with tape measures and pins, stitching luminous plastic cherries to green cloth leaves and fitting bridal veils. There used to be seven or eight workers upstairs, too. Now Washington's oldest milliner handblocks hats alone.

Bare bulbs reinforce the weak light trickling in through the barred, dusty-gray window. With its massive hat press, old sewing machine and porcelain basin, the room resembles an old garment-district shop. Not much has changed since the days when Bachrach, his brother Nathan and the others bustled to turn out Panamas to a clamoring public.

But Bachrach seems to like ghosts. He's surrounded by hundreds of them: They are wooden and bear the shape of every hat he's ever made. They're called hat blocks, and on their smooth surfaces Bachrach molded cloth into his stock in trade. The hats were then steamed, dried and sent downstairs to the women for trimming. There are huge turn-of the-century "merry widow" brims, conical shapes from the '50s and timeless berets of all sizes. The blocks are hand-carved and valuable, yet Bachrach won't sell them. He says the styles might come back, but each wooden head is a piece of a past Bachrach won't part with. Nor will he part with his 11th Street shop.

"We were invited to come into the shopping centers when they first opened. But we can't be out there. We have to be centrally located," he insists. But most of Bachrach's former customers don't shop downtown anymore. In fact, Bachrach credits his survival to downtown's present shoppers - black women. "Black women have kept up the tradition of wearing hats to church, and they come here because they know we'll make them exactly what they want. Fur hats have helped, too. We make them and in the last 10 years they've gotten very big. I've seen the worst. But I think young women are beginning to realize that there's something missing when they dress up, and that missing link is a hat."

Mrs. Sheftell, too, admits that business this year is much better than last year. "The young girls come in here and tell me that they're buying their first hats. And they're so excited. I remember when they used to walk on the Avenue in torn jeans.Now they're dressing up again. It makes me feel wonderful." CAPTION: Picture, MILLINER SONIA SHEFTELL, OWNER OF THE REGIS ROOM, WITH SOME OF HER HATS. By Gerald Martineau.