Had the chance to head off to Downtown Anaheim this afternoon, and did some serious research, both at the Main Anaheim Library, and the Heritage Room at the Museo. While a lot of the info does focus on the fact that Disneyland officially celebrated opening day as July 18th, 1955 when Walt Disney was alive. I have a Disneyland Press Release from July, 1967 showing that Opening day was July 18th (and Walt Disney passed on in 1966).
And while there is a lot in regards to opening day, there is a lot of other fun stuff!
And yes, July 17th is STILL an important day, as the 1967 Press Release calls it "Dedication Day".