The Disney Annual Report for 1969 lists the opening day of the Haunted Mansion as August 12.
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Disneyland Inside, vol. 4, no. 9 (September 1969)
As you can gather from some of those clippings, there was a
"sneak preview" for the press at midnight, Monday the 11th/Tuesday the 12th, marking the official opening of the Mansion. A large number of press reports over the next few days refer to it.
This is a pretty crushing set of documentary evidence. There is also some anecdotal evidence, like this note to Jeff Baham at Doombuggies.com:
No one would have been freaked out about the Manson murders in time for an August 9 grand opening. The Sharon Tate murders took place that very morning.
So where did the August 9 date come from? From Marc Davis, apparently. The can of worms spreads out like this. There was indeed a Cast Member preview on the 7th and 8th, a so-called "soft opening." There was also the Monday night/Tuesday morning press preview, marking the official grand opening at midnight. These were planned and announced in advance. But according to "Todd Hackett," who worked for Marc Davis many years and was around for the Mansion's debut, Marc took out ads in the LA Times announcing that the Mansion was "now open." This full-page ad ran on August 9:
Todd Hackett saved a copy of that ad, plus an ad that ran in the Calendar section of the LA Times Sunday, the 10th, and this one is our smoking gun, since the "August 10" date is printed on the ad itself.
According to Hackett, Davis felt the public had waited long enough and decided to pull in one more weekend for the Mansion before the summer was over by opening a few days early. His impatience with the long delay is manifest in the full-page ad, which he drew himself:
However, there is other testimony claiming that Davis did this reluctantly,
acting in response to pressure on him from Disneyland staff:
Update Dec 20, 2015. I've been corresponding with a former WDI Imagineer (known as "gerG"; his friends will recognize the name), and he has some further information to shed on this period:
"I've always heard odd stories about how the Mansion opened. It was an odd, transitional and difficult time for the company in 1969 for management and designers. The company was incredibly profitable, but without Walt, focus was blurred. I know that Marc stepped up and took control of the Mansion, both at WED for design and manufacture, and D'land, for construction, installation and operation. He was at the park a lot to supervise, which was slightly odd, but needed.
Remember also that the construction schedule for the Mansion was strange. It wasn't planned to open in the beginning of summer, when operations would want. Perhaps it was planned to open in October, which would make sense. Either way, installation went well, and they were testing the ride system (without passengers) in late July."
"gerG" also has information from a friend who knows Alice Davis:
"I spoke to him about the date of the opening. He's rather close to Alice Davis, and he spoke to her last week [i.e. early Dec 2015] about Marc's decision to open it. Alice said that yes, it was Marc who decided to open the mansion, but he was being pressured heavily by d'land operations. He was in the middle of testing and adjusting, and Alice said that he really wanted more time to change things (like the Hatbox Ghost, which didn't work), but he bent to the pressure and agreed to the opening. And remember that there was no press opening...no previews...and almost no advertising. Even those radio spots seem rushed. It was a strange time."
People who saw the ad or who simply happened to be at Disneyland that Saturday found the Mansion open for business. As gerG says, there was no fanfare, no announcement—nothing. Davis had it opened despite the press "sneak preview" scheduled for Monday night, and despite the fact that there were apparently still some nagging problems with the ride. We know that Cast Members spotted problems with the Hat Box Ghost during their soft opening, and mere hours before the Saturday opening he was pulled out (the Hat Box Ghost, not Marc Davis). During the press event on Monday night there were sound problems, and Disneyland Ambassador Shari Bescos had to stall the reporters until the problem was fixed as they made their way over to the Mansion from Club 33 in New Orleans Square, where the event began. There is
circumstantial evidence suggesting that the hitchhiking-ghost-in-mirror gag was only jury-rigged for the press event and the backup effect was put back in place sometime during the 12th, which remained on the books as the official grand opening day.
Problems and all, the early opening worked. Word spread quickly. This is what Disneyland looked like the following Saturday, the 16th. Park attendance (82,516) set a one-day record that stood for years and years.
As for the question in our title, that chamber has no windows and no doors. Either the Mansion had
two official grand openings (which is nuts), or it had one official opening and one unofficial "soft opening" to the public, except that the soft opening was announced in the newspapers, which makes it, um, official, doesn't it? An advertised soft opening—yeah, that's also nuts.
Aieeee, I've got me some cognitive dissonance going on here. I too feel the disturbance in the Force, but I cannot help you. You'll just have to decide which absurdity is the less intolerable, pick up the shattered pieces of your life, and move on.
In truth, the fact that the Haunted Mansion had a debut that defiantly and definitely defies all definition only makes it more fun.