It happened on May 4th, 1977 – Disneyland opened Space Mountain – though not ‘officially’… The attraction was open to guests to ride for an “E” ticket, but the park’s second ‘mountain’ wasn’t dedicated until a few weeks later on May 27, 1977.
It’s also interesting to note that Disney World opened their version of Space Mountain in 1975 – BEFORE Disneyland! Though Disneyland’s version is only about 2/3 the size of the one in Florida.
Disney Imagineers came up with a version of high-speed roller coaster in the 1960s, but construction of the Mountain didn’t start until June of 1975. It’s 173 feet tall and with the spire, it’s the highest point in the park.
In 1993 Federal Express started sponsoring the space port and displayed signs with a twist on their original slogan. Instead of “When it absolutely, positively has to be there overnight.” it was something like “When it absolutely, positively has to be there in .5 seconds.” In April of 2003 FedEx dropped their sponsorship.