Rehab is more than one of the most infamous parties and best-known brands in the history of the Las Vegas club scene. The mere mention of the Hard Rock Hotel’s longtime summer bash evokes images of crowded swimming pools and bikini-clad bodies moving with the music in an effort to pack the greatest Spring Break ever into a single sun-drenched day. Buckets of icy beer surround daybeds and bottles abound for those with cabanas, including the hottest celebrities of the day. Anyone who’s been to Rehab has a story to tell.

But now, on the occasion of the grand-opening launch of the pool club’s 15th season—featuring Flo Rida on Saturday’s Cinco de Mayo party and DJ Whoo Kid on the always-industry-oriented Sunday—it’s time to recognize Rehab for something more than all that summer fun. This is not just a party pioneer, but a wild experiment that changed the face of Vegas forever.

Whether you’re trying to change the game the way Rehab did or maintain a hot venue or event, rule No. 1 is simple: The people make the party.

“Rehab and the Hard Rock have always had really passionate people that carry on the party, year in and year out,” says Joe Bravo, director of nightlife and daylife for Hard Rock Hotel the past four and a half years. “When I came in, there was a robust brand handed off to me, and every time that’s happened over the years it keeps getting stronger and stronger. Different clubs have different priorities. We’re owned and operated by the Hard Rock and not a third party or different ownership group, so the Hard Rock has that vested interest in making sure it’s a good party and all good for our guests.”

In 2004, Chad Pallas was the Hard Rock’s director of nightlife. He’d been at the hip property for nearly six years, and changes were being made: Baby’s nightclub was becoming Body English, and a new staff was coming in. Thinking he needed a new focus to stay relevant during the transition, Pallas concentrated on the pool and an idea spawned from weekly boating trips to Lake Mead.

“It was the greatest time ever to get out there in that cheap pontoon boat and party during the day,” Pallas says. “I had just come home and was relaxing one day when I got a call from a girl who wanted to get a group of her friends into the resort pool. She said she couldn’t just walk in, that they had grown more strict, yet I was going to meetings where we were complaining that pool business was down.”

Pallas put his day-party habit together with the demand for poolside fun to get to the idea for Rehab, but he wasn’t convinced it would be a success in Las Vegas. “I had seen Nikki Beach and thought it was an interesting concept. I liked it, but I didn’t think what worked in Miami would work in Vegas. We had to think, what would this be in Vegas?” he says. “At first we made the mistake of thinking it was more of the electronic [music] scene, but the crowd that came showed us it was more of a Spring Break party scene.”

He got the green light based on what seemed like a risky claim. The Hard Rock pool was making $10,000 on Sundays. Pallas promised he could double that. “But I was thinking, I hope I can do it,” he says. “The first Sunday, we did $90,000.” In its first summer, Rehab grossed $1.5 million, according to Pallas.

With all the luxurious dayclubs and pool parties that crowd the Strip’s casino resorts these days, it’s hard to remember that this was a nonexistent form of entertainment here just 15 years ago. While part of Rehab’s fast success stemmed from its lack of competition, it truly exploded by focusing on providing a great party for local nightlife industry and hospitality workers. As they checked in, word of something new and wild just off the Strip began spreading to the Vegas visitors with whom they regularly came into contact.

“That’s one thing about Rehab a lot of people don’t remember, that it started out as a locals’ party,” Bravo says. “It’s always been the place for locals who’ve been working their butts off all weekend to come and blow off steam and relax. Without that local flair, it’s not the same, and we’ve had periods in our past when we catered to them more or less, and you can see the dip in the party. We’ve realized the locals make the party.”

Once the Rehab party was made, celebrities flocked to the Hard Rock pool to watch the show. From Kim Kardashian to Paris Hilton, Puff Daddy to Dennis Rodman, Lindsay Lohan to Justin Bieber, all the famous folk who had a reason to be in Las Vegas made sure to stop by—and they were there to party. And then there’s the countless superstar DJs who cut their teeth spinning at Rehab.

“Rehab is an institution,” says local favorite DJ Five. “It started all this, and it’s still one of the best pool parties in Vegas after all these years. It’s always been one of my favorite parties to play.”

Because of all the traffic generated by the event, Sunday nights became the second-highest-grossing day for the Hard Rock’s casino, restaurants and occupancy, according to Pallas. The original $20 admission eventually increased to $50, because the lines to get into Rehab had grown so long. But even after that, there were thousands packing the pool and a few thousand more inside waiting for their chance to join the revelry.

At the risk of stating the obvious, it should be mentioned that the perception of naughtiness plays an essential role in the Rehab mythology. Any nightclub in Las Vegas or any other city has to handle the potential perils that come along with the business—sex, drugs, people partying too hard. As the first daytime party of its kind, Rehab might have looked and felt like a place where one could get away with anything, but that was never the case.

“It’s dresses at night versus bikinis at the pool, and the bikinis just look more risqué, so it felt crazier to people,” Pallas says. “We were always under so much scrutiny. The sheriff’s office was down there every week. I still know [former sheriff] Bill Young, because they were just all over us.”

As Vegas pool parties and dayclubs have evolved, security and safety procedures have become even more enhanced. But in terms of promoting the party, walking that tightrope is just the Vegas way. “It definitely makes life interesting,” Bravo says. “People sometimes expect to be able to do anything, and while you want to have that image of freedom, you can’t let that happen. There were things that could fly in 2004 that you just can’t do now. It’s really more about an escape and leaving your inhibitions at the door and having fun, and for us, throwing a good party and getting wild and crazy while still keeping everyone safe and responsible.”

The potentially debaucherous and occasionally drunk aspects of the party were promoted in three seasons of the TruTV series Rehab: Party at the Hard Rock Hotel, which aired from 2008 to 2010. The show followed the staff and patrons and purported to be a “docudrama,” but it was closer to a scripted reality show.

“I do believe that getting a million people to view the party on a weekly basis on TV really extended the lifespan of the party,” says Matt Minichino, the Hard Rock’s nightlife boss from 2009 to 2011.

By that time other dayclubs and pool parties had popped up, including Wet Republic at MGM Grand and the weekly Ditch Fridays event at the Palms. Today’s dominant venues—including Encore Beach Club, Marquee Dayclub, Daylight, Tao Beach and Drai’s Beachclub­—incorporated lessons learned from Rehab’s development and took Vegas daylife to the next level.

“If you’re choosing between daylife and nightlife, I think daylife is the most popular form here,” Minichino says. “The Hard Rock really was the trendsetter of Vegas daylife, but when you’re doing three, four, 5,000 people, you lose a lot of the luxury experience. It paved the way, and then you have people like Alex Cordova [currently at Wynn Nightlife] and Sean Christie [currently at MGM Resorts] who have mastered high-volume daylife and maintaining that luxury, VIP experience and venue.”

Despite competition and programming by its host resort, Rehab is still here … for now. This 15th-anniversary season should be fun as ever, but with the Hard Rock under new ownership and planning to rebrand the entire property as a Virgin Hotels destination, this could be Rehab’s last summer.

“We know Rehab is a big part of this property, but how it evolves from here, I don’t know,” Bravo says. “My mandate from the new owner is to make it fun, and that’s what we’re going to do, week in and week out—throw a killer pool party. I think this is the year Rehab re-establishes itself in the pantheon of great pool parties we have here in town.”