One would expect Pirjo Jaffe to be skilled with knitting needles; she started to knit at age 5 and "I was good at 7." Knitting was a part of nursery school training and in elementary school Jaffe was making potholders, scarfs and hats.

"In Finland, winters are long and dark; people stay inside and do handwork." And when knitting was no longer part of the curriculum, she would knit under her desk, making complicated sweaters for her friends.

It's hard to imagine those sweaters being more intricate, more attractive than those she makes today. "I used to have more time and I would just knit for the fun of it. Now it's a business."

Business or not, the reembroidered sweaters, the chenille coats, the cocktail dresses she does now are inventive and difficult to make. "In truth," says Jaffe, "if I got paid for the time it really takes . . ." She doesn't finish her sentence. "Some of the coats take four to five weeks to make." And it takes days just to weave in the ends of the embroidery on the coats.

Jaffe's mother did beautiful knitting and crocheting, she recalls. "Her technique was superb, far better than me. But for her it was work, for me a great pleasure." Jaffe takes her knitting to the theater, to friends' houses, even to the beach. She doesn't have to count stitches; she doesn't have to look at her work.

"Like the ad says," she teases, "I never leave home without it."

Actually she is setting up shop outside her home, in downtown Washington. Her things have been sold at I. Magnin and Liberty in Georgetown, but by mid-February she will have her own shop upstairs at 1332 G St. NW. She will sell clothes by Marimekko but mostly her own knit and crochet designs, all in the imported hand-spun wools she prefers. "I'm a little scared. Washington is so conservative. The women in Potomac seem to understand my things and seek out things that are different and special, but I'm less sure of the rest of the city."

Once she opens her shop, she is bound to be pleasantly surprised. Showing the Fabric Of History

Samples of fabric, no matter how beautiful they are, hung flat against the wall make an exhibit of limited appeal. But, when the exhibit is woven with art and history and life style, the fabric pieces become another way of learning more about ourselves. That is always the joy of exhibits at the Textile Museum.

Now there is a small textile show at Meridian House, a traveling exhibit of the Smithsonian Institution -- "Five Centuries of Italian Textiles" -- worthy of a visit. Representative textiles from 1300 to 1800 are hung on panels with examples, unfortunately none original, illustrating the way the same designs were used in painting, sculpture and architecture during the Middle Ages and Renaissance.

The remains of the textiles, all very sumptuous, represent the peak such artistry in Italian fabrics. The textile industry was created to meet the demands of the aristocracy, the wealthy bourgeoisie and the church. It took months to weave the fabrics of silk, gold, silver and expensive natural dyes.

The show, which will remain open to the public until Feb. 3, is well suited to the Meridian House, a fine example of 18th-century French urban architecture. Says Nancy Matthews of Meridian House, "The show had a special meaning for me. It gave me new respect for some of the wallpaper in my hallway, which has the same pomegranate motif as in one of the fabrics."

The show's curator, Rosalia Bonito Fineli, an American who has been associated with the Museo del Tessuto in Prato, Italy, which provided the fabrics, has also done the very worthy catalogue. According to Fineli, who has lived in Florence for more than 20 years, the bright pink and green colors used in a 15th-century silk cut-velvet shown in the exhibition are the very same tones on the planning boards of wool mills in Prato for the future.

Maybe so. But there is one influence from the show likely to show up long before then. Bloomingdale's interior designer Mark Quellette covered balls in brocades and damasks to decorate the Christmas tree that still stands outside the exhibit. He also hung tassels from the tree. It's the tassels that are likely to be the incoming detail with the trendy set, hung from handbags and hats as well as a decoration for the hair. Photography That Excites

Fashion photographer Helmut Newton has spent the last week in Palm Beach, Fla., "working on my Monte Carlo tan," he says. He's never been to Washington, D.C., before and he wants to know what the weather's been like.

"Oh, that's not good," he responds nervously. "I don't own a coat. I live in Monte Carlo and spend the winter in L.A."

This year marks his 50th in fashion photography, a career he started in Berlin as an apprentice. More than any fashion photographer, Newton brings the underlying seductive current in fashion photographs to the fore. Magazines still get letters complaining about his pictures, he says, and it doesn't seem to bother him. "I think fashion has a lot to do with eroticism. I did a lot of things which were quite daring, which are nowadays nothing."

Now, Newton spends most of his time taking portraits, which is why he'll be freezing in Washington tomorrow -- to give a slide lecture for the Smithsonian Resident Associates. He won't buy a coat, and he insists, "I'll stay inside and take taxis."

"Clothes can inspire me, the place, the women can inspire me, but with portraits," he adds, "certain people excite me, fascinate me. David Bowie was wonderful, wonderful, as was Elizabeth Taylor" (whom Newton photographed for the December Vanity Fair cover). "Anthony Burgess was terrific . . . but all you wanted to do was listen to the man. It's a waste of time photographing him."

The portraits are no less unusual than his fashion work. He likes taking group pictures, "where there is a relationship to show, it's much more interesting," and his recent portrait of designer Azzedine Alaia, whom Newton has known for years, included some of Alaia's friends.

Who was Newton's first portrait subject?

"Myself, with a self-timer."

The Helmut Newton lecture for the Smithsonian Resident Associate Program will be Monday at 8, in the Baird Auditorium of the Smithsonian's National Museum of Natural History. Nonmembers are welcome. For details, call 357-3030. -- M.S. Dailey Wrong Wear for Skiing

For Donna Karan, the biggest injury on her skiing vacation in Aspen over the holidays was to her ego. "The instructor kept insisting I should be wearing bell bottoms," said Karan, who refused to give up her stirrup pants despite the blisters they caused on the bottom of her feet. "I'm sure it was the boots, not my pants," laughed Karan.

For Calvin Klein, his ski injury was no laughing matter. "There's probably nothing wrong," insisted his ski instructor, who encouraged Klein to take one more run in Cortina, Italy. But the doctor had another view. This week Klein gave up the hip-to-ankle cast for one that goes only from ankle to above the knee, but he still needs his crutches. Meanwhile, he's planning a ski vacation for March. "I'll never learn . . ." he said with a grin.