I saw somewhere that it's April 21st, but I can't find anything on SFMM's site to verify. On the site, it just says that it will open at a later date this spring.
One of the managers at our ride preview yesterday seemed certain that April 21st was the official public opening date. I haven't seen any media on this though, which is surprising because you would think their advertising department would want to give the public at least a months notice in advance to generate hype and get folks planning trips. Idk, I speculate the tech rehearsals for the VR could possibly make setting a public opening date in stone a little sketchy.