The pair met in 1978, when Elkins was an official in the White House of President Jimmy Carter. Archibald, an artist, came with some friends to deliver a painting.
"It was love at first sight," says Elkins.
Ten years later, at the anniversary party by the pool, a friend asked whether they should bring presents. No, said Elkins. Bring money to donate to two HIV/AIDS service groups in Washington and Baltimore.
Like the couple, the event has endured. Last year drew about a thousand people and organizers expect this weekend to be just as packed.
CAMP Rehoboth, or Create a More Positive Rehoboth, is dedicated to improving the relationship - and life generally - between the straight and the gay, lesbian, bisexual and transgender communities in the coastal city.
These days, Rehoboth ranks fourth in the country for its per capita number of same-sex couples who are residents, according to one study. City and police officials chortle about the town's diversity.
"That's what makes it a great community," says Police Chief Keith Banks.
But 25 years ago, Rehoboth wasn't so welcoming.
Elkins, executive director of CAMP Rehoboth, remembers seeing a bumper sticker in 1990 from a local homeowners group.
" 'Keep Rehoboth a family town.' We knew what that meant," he says.
It meant, at the least, ignorance. The worst occurred in 1993, when five males seriously injured three gay men by beating them with champagne bottles and a baseball bat.
Archibald, who will be 58 on Friday, and Elkins, 62, had established CAMP two years before that incident, when Rehoboth Beach became known as a gay destination.
The problem was, "the town itself hadn't adjusted to being a gay destination," says Archibald.
The beating was a galvanizing moment.
"It was really a bad incident," Elkins says, "but it helped focus things."
CAMP Rehoboth - the name plays off the town's beginnings as a Methodist camping retreat and a gay sensibility of campiness - provided a gathering place for the LGBT community and an information center for straight neighbors wanting more information.
In the beginning, CAMP was a mediator between the gay and straight communities. Archibald and Elkins expanded its reach by adding advocacy work, a magazine, a women's golf league, grief counseling (many members of the gay and lesbian community, including some of the couple's friends who were at that first pool party, died of HIV/AIDS), support groups for substance addictions, and sensitivity training for police.
Banks was an officer on the force when the training started. Now, he and Elkins lead the training for the extra officers hired each summer. Role-playing was the early technique; now, the training is more of a conversation and a chance to ask questions.
"There's a generational shift that has been so amazing," Elkins said.
Banks says he and CAMP have a strong rapport and share information. Three years ago, Banks called Elkins late one night to tell him that neo-Nazis in nearby Ocean City, Md., had been overheard saying they were going to come to Rehoboth Beach to clash with gays after the bars closed. Banks wanted to let Elkins know that police were monitoring the group, and Elkins had bars warn patrons. In the end, the group never went to Rehoboth Beach.
The CAMP center has a meeting room that is used by straight and gay residents alike, for everything from civil union ceremonies to cholesterol screenings. There is gallery and performing space.
"It has really become part of the entire community in a very natural, educational progression," says Rehoboth Beach Commissioner Patrick Gossett.
Even as Rehoboth Beach has changed and CAMP has grown, some of the guests this weekend will remember Sundance's original purpose.
"People still wish us happy anniversary," Archibald says.
For information on Sundance, go to www.camprehoboth.com or call 302-227-5620.
Contact Carolyn Davis at 215-854-4214, firstname.lastname@example.org, or @carolyntweets on Twitter.