(CNN) -- Thomas Corless has been to Walt Disney World in Orlando, Florida, 101 times.
"The Disney Parks are unlike anything else on the planet," he said.
Corless, who runs the Disney fan site WDWNT.com, will be in Orlando this weekend to celebrate the theme park resort's 40th birthday.
Since its opening on October 1, 1971, Walt Disney World -- or WDW, as it's often referred to online -- has become the world's most visited recreational resort. It's also more than twice the size of New York's Manhattan island.
People get married there, they go there on their honeymoons (more than anywhere else in the continental United States), and of course it's a major destination for families.
For those who go over and over again, like Corless, there is a big nostalgia factor. He counts attractions that no longer exist at the park among his favorites.
"I think the fascination is partly nostalgia for characters & childhood memories and partly the promise of fun," the iReporter said. "You know when you go to Walt Disney World, you are going to have fun. There aren't many places you can take the family and have that guarantee."
For other repeat visitors, like Pete Werner, a visit to the resort can mark an important point in their lives.
"I was newly sober and only had one day to spend at Disney World while in Orlando for business [in 1993]," he said. "My first park was Epcot, and as a result of that visit, I became enamored with everything Disney. I promised myself for each year I stayed sober, I would reward myself with a visit to WDW."
Werner doesn't think one thing in particular makes the parks so special. Like him, many who visit the parks have one special memory they take with them.
"Everyone has a story -- a memory -- that they relive every time they return," he said. "Disney is really its own world -- and a place where you can escape from reality without any of the negative consequences that usually come with other forms of escapism."
Ryan Wilson, another hard-core fan, iReporter and creator of Main Street Gazette, said he remembers specific moments instead of rides or attractions: "Sharing a great meal with a great collection of friends, watching my wife's face light up as she watches fireworks burst overhead, or wandering old stomping grounds with my mom and dad. Walt Disney World is, for me, the magical place that means so much to my friends and family, and they are the heart of my experiences there."
Just before packing for his 102nd trip to the parks, Corless said that he believes Walt Disney World is "unlike anything else on the planet. Sure, there are other theme parks, but they aren't even close to Walt Disney World in popularity, quality, and most certainly in meaning to American culture."
For hard-core fans like Corless, Walt Disney World history is something that is discussed on the many websites devoted to the parks. Here are some important dates in WDW history:
October 1, 1971: Nearly five years after the death of Walt Disney, the resort bearing his name officially opens in an area that used to be swampland, with Magic Kingdom theme park attractions like "It's a Small World," the Haunted Mansion, the Country Bear Jamboree and the Mad Tea Party. The resort surrounding the park would bring in tourists from all over the globe over the next 40 years.
December 15, 1973: Pirates of the Caribbean, one of the most popular rides of all time, opens in the Magic Kingdom's Adventureland.
January 15, 1975: Two centerpieces of Magic Kingdom's Tomorrowland -- the Space Mountain roller coaster and the Carousel of Progress show -- open to the public.
June 11, 1977: The first Main Street Electrical Parade takes place in the Magic Kingdom, state of the art for its time.
October 1, 1982: Walt Disney's vision of the future, EPCOT Center (consisting of Future World and World Showcase), opens with attractions such as Spaceship Earth, The Land's Kitchen Kabaret and World of Motion. Fan favorites Journey into Imagination (introducing the original characters Dreamfinder and Figment) and Horizons would open within the year.
September 12, 1986: Michael Jackson stars in "Captain EO," which makes its world premiere at EPCOT Center. (It would return to the park in 2010, in memory of the pop star, after a 16-year absence.)
May 1, 1989: A third park, Disney-MGM Studios (which would herald a new spate of filmmaking in Orlando -- at least for a short time) opens. "Star Tours," the "Star Wars" thrill ride, would follow in December.
October 2, 1992: Thrill ride Splash Mountain makes its debut at the Magic Kingdom's Frontierland.
July 22, 1994: The "Twilight Zone" Tower of Terror makes its worldwide debut at Disney-MGM Studios. Thrill-seekers line up to take the plunge.
April 22, 1998: A fourth park, Disney's Animal Kingdom, focused on nature and conservation, opens to the public. A roller coaster, Expedition Everest, would be added in 2006.
September 7, 1998: Mr. Toad's Wild Ride closes. Fans protest with a "Save the Toad" campaign that doesn't pan out. (The ride is still going strong after 56 years at Disneyland in Anaheim, California.) The Many Adventures of Winnie the Pooh ride takes its place in the Magic Kingdom's Fantasyland on June 4, 1999.
October 10, 1998: Journey Into Imagination closes, to be reopened as a completely new ride, Journey Into YOUR Imagination, on October 1, 1999. The ride, which contains very little of the character Figment, and no Dreamfinder, is poorly received, and closes on October 8, 2001, only to return with major changes as Journey Into Imagination with Figment in 2002. However, some hard-core fans have campaigned to bring back the original ride, or at least Dreamfinder.
December 19, 1998: Epcot (no more "Center" in the name) opens its first thrill ride, Test Track, replacing World of Motion.
August 15, 2003: Mission: SPACE, a thrill ride simulating space travel, takes the place of Horizons at Epcot.
July 7, 2006: Audio-Animatronic figures of "Pirates of the Caribbean" stars Johnny Depp and Geoffrey Rush are added to the original ride of the same name.
July 4, 2009: A new version of the Hall of Presidents show opens at the Magic Kingdom's Liberty Square with the addition of President Barack Obama.
2013: New Fantasyland, an expansion of the area in Magic Kingdom, is set to open, replacing Snow White's Scary Adventures with a Seven Dwarfs-themed mine ride, and adding a new version of Dumbo the Flying Elephant, as well as the Beast's castle from "Beauty and the Beast." Construction is also scheduled to begin on an "Avatar"-themed land in Animal Kingdom.