Every Thursday in recent weeks, 73-year-old Helen Harrison Weik has trooped into the Knock on Wood tap dance studio in Silver Spring. Ever since she received a diagnosis of terminal cancer last summer, Weik has been determined to refresh her tap-dancing skills and to put on a demonstration for her patients at the special Christmas show at the Veterans Affairs Medical Center in Northwest Washington.

The show is Thursday, and Weik will dance to a military medley of "Battle Hymn of the Republic" and "Anchors Aweigh." It will be the first time she has tap-danced in public since she was a little girl, when she performed for Fiorello LaGuardia, a congressman who would later become a legendary mayor of New York. And it will be evidence of the power of setting goals, testing limits and living each day as though it's your last.

Weik is a nurse at the VA Hospital, and she has colon cancer. When she was operated on in early July, her doctors found that her cancer had spread to her liver and lymph nodes. She refused the chemotherapy and radiation she was offered, and doctors told her that she had one to six months to live.

The diagnosis gave Weik a new philosophy, an eagerness to do things she always wanted to do before it was too late. "The kid in me is having fun," she said about her weekly tap sessions with Renee Kreithen, co-director of the Knock on Wood studio. "I'm doing things I never had a chance to do."

Shuffle, shuffle, shuffle, and step. Shuffle-ball-change, scuff, bend, step.

Scientists have shown that being socially engaged and having an upbeat attitude like Weik's helps improve a patient's odds against serious illness. And from her background as a psychiatric nurse, Weik herself knows the healing power of having a clear, important goal.

Her first goal is to get through Thursday's performance. Weik said she suffers from stage fright, but in rehearsal with Kreithen, she looked fearless. She kept speeding up the tempo, eager to get going, to get it right, to cram in everything that's yet to come in the time that remains.

A slim woman with a blonde bob and a smile that lights up a room, Weik tapped insistently across the floor, holding Kreithen's hand. She held herself very straight, her right hand in Kreithen's, her left hand gracefully poised in midair.

The shuffle steps that make up most of Weik's routine were tapped out nice and loud, and when she did her "ball-change" steps, she gave them a percussive little hop that dancers half her age can have trouble with. By the end of the hour-long lesson, Kreithen, 48, could barely keep up.

"Hey, Helen," she asked good-humoredly. "How come I'm huffing and puffing--and you're not?"

Her health problems have not slowed Weik down. She still works five days a week, and she recently spent her day off baking cakes and cookies for the party after a Knock on Wood performance. She says she gets tired at about 3 in the afternoon, "but so does everybody." When asked how she's doing, she says, "Fine--a little bit of diabetes, a little bit of osteoporosis, a little bit of cancer, but fine."

After the Christmas show, Weik said her next goal is to find herself a man.

Weik was married twice, and each time her husband "passed on--to another woman." Since her diagnosis, she has gotten serious about looking for a companion. She went to a dating service, which offered to set her up with a man they admitted was "sort of old--he was 79. But I said to them, 'I'll take them up to 85!' "

Shuffle-ball-change, scuff, lift, stop. Shuffle-ball-change, step.

When Weik danced for Mayor LaGuardia, she was 5 years old and recuperating from pneumonia at the New York Foundling Hospital. She and her two younger sisters were living at St. Agatha's Home for Dependent Children in upstate New York. Their father was dead and their mother was dying, and the girls spent their childhoods in foster care and St. Agatha's.

"I guess it's really a Tiny Tim story," said Weik, who does not dwell on her hard-luck beginnings.

One thing she carries from those days is a special love of dolls. She was given a "Patsy" doll, a baby doll popular in the 1930s, when she was in the hospital, and she slept with it every night. But when she was sent back to the orphanage, she was not allowed to take the doll. She grieved long and hard for that toy. As an adult, she began collecting dolls and now owns more than 200.

At the age of 38, a single mother with two young daughters, Weik enrolled in college to become a nurse, then went on to get a master's degree in psychiatric nursing.

Shuffle-ball-change, shuffle, lift, step. Shuffle-ball-change, step.

The VA Hospital's Christmas program has been organized by Waltraud Elisabeth Larson, the hospital's creative arts therapist. Performing "gives the men a little boost," Larson said, "and makes them feel important again."

Weik will be the only staff member in Thursday's lineup. But she is more than just a caregiver at the hospital--she has become the ward's unofficial "sweetheart."

"It's like in wartime, when they had pinup girls," Larson said. These days, Weik dispenses hugs to all the men, most of whom are aged and severely disabled. The affection she dishes out is therapeutic for her as well as for her patients.

Larson said she is not surprised that Weik has turned to dancing during the final stage of her life. "I firmly believe that the arts are within each one of us," she said. "To complete our life, we need to find that part."

Learning to tap again has also given Weik a circle of new friends, starting with her teacher. Kreithen recently brought Weik along when she visited her father at a rehabilitation center. She said she knew Weik's smile would lift his spirits and hoped the visit would lift Weik's as well.

"My father told me to pay very close attention to Helen during our lessons," said Kreithen, who is giving Weik the lessons for free. "He said I would probably learn as much from her as she would from me."

For Weik, the Christmas show will be a beginning, not an end. "I was never supposed to live this long," she said. "But I believed I would. I told my doctor that if I made it to Christmas, I would buy him a pair of Salvatore Ferragamo shoes." She plans to start shopping right after her performance.

CAPTION: Helen Harrison Weik, right, rehearses her tap dance routine with her instructor, Renee Kreithen.