The event is set in the 1890s when Pleasanton was one of the most desperate towns in the West.
"When I was researching downtown, almost everything seemed to be a brothel or a bar," recalled board member Rebecca Bruner, who came up with the idea for a fundraiser. "I thought to put it in the time of Phoebe Hearst makes it kind of fun."
The main hall at the Senior Center is being transformed for the evening.
"We put up false front buildings and cover the walls so it looks like you're in a saloon," Bruner said. "There's entertainment all night, dancers and singers."
"We'll also have gambling going on all night -- blackjack and craps," she added. Volunteers from the Lions Club are handling the gaming tables.
Attendees must leave their horses and weapons outside, Bruner cautioned.
"Pinkerton men will check you," she said. "Sen. George Hearst and his wife Phoebe are the host and hostess. George is trying to get out the vote so they rented the place for citizens to come out."
She also pointed out that the Pinkerton men will act quickly to keep in line bandits or other renegades that crash the party.
"There are a lot of little things going on," Bruner said. "We try to talk like it's 1890."
Museum volunteers dress the part, she explained, but everyone is welcome, in costume or not.
"A lot are dressed up, a lot aren't," she said.
When folks buy tickets they can get suggestions about characters to play.
"Pleasanton wasn't always a pleasant town, and it certainly wasn't pleasant when we had brothels and characters wandering through," Bruner said.
Downtown was also home to brothels and bars during World War II, she noted, and the abundance of bars remained into the 1950s.
"Older people who lived here said they didn't watch TV on Friday nights, they went downtown and watched the fights," Bruner said.
This fundraiser is a chance to see Pleasanton as it was in the 1890s, when "it was at its wildest."
"It's a way of raising funds and teaching people a little bit of history while having fun," museum Executive Director Jim DeMersman said.
Museum On Main has an annual budget of $232,000. Its major source of funding is the city of Pleasanton, he explained.
"We're a private nonprofit, and city support is crucial to our survival," he said. "It's what's allowed our Board of Directors to hire professional staff."
Other money comes from the museum's two annual fundraisers -- Brothels, Bar Rooms and Bandits; and the Ghost Walk -- plus its membership program as well as grant writing.
The Ghost Walk is a series of downtown walking tours on Friday and Saturday evenings on the last two weekends in October.
"'Ghost hosts' take people around, and there are some cool stories," DeMersman said. "It's another fun way we teach Pleasanton history, by talking about ghosts."
Attendance at the Museum On Main was up 69% in 2010, and DeMersman said it's drawn more visitors with the recently extended hours, which include Tuesdays and opening an hour earlier, at 10 a.m., Saturdays.
Tickets for Brothels, Bar Rooms and Bandits are $40 per person, which includes a barbecue dinner, gambling chips and "the best entertainment in the West." Cash, credit cards and gold nuggets are acceptable.
But, warn the organizers: "Over 21 only -- or we'll call the sheriff."
Visit the Wild West saloon
What: Brothels, Bar Rooms and Bandits
Who: Museum On Main
When: 6-10 p.m., Saturday, Sept. 24
Where: Pleasanton Senior Center, 5353 Sunol Blvd.
Tickets: $40 per person; call 462-2766 or go to the museum at 603 Main St.