On the 17th vs. 18th thing. Actual historical dates and the celebrations of the events that supposedly took place on those dates are frequently at odds. Jesus wasn't born on December 25th. We celebrate Lincoln's birthday on a different day than the real one. And so on.
The Haunted Mansion officially opened to the public on August 12, 1969, and NOT on August 9th, as virtually all sources tell you.
In the end, it just means you can have TWO parties if you want.
(Just saw CU's post. Said it better than I)