Probably coming worldwide shortly after the Chicago GO fest, as I imagine them unlocking it is pretty much a foregone conclusion. Though it seems like Legendaries will be releasing gradually into the raid pool. I'l also heard scuttlebutt that Legendary raid eggs will have an extra-long lead time, to make it easier to coordinate a large group, though I'm not sure where that info comes from.
Someone over on the Silph reddit also did a pretty nice write-up on which pokemon that you can raise now will be most useful for Gen 1-2 Legendary raids: https://www.reddit.com/r/TheSilphRoad/
For me, right now I've got one Bite/Crunch Tyranitar and one Rock Throw/Rock Blast Golem, plus the usual complement of Eeveelutions and one each excellent Scizor w/Bug, Houndoom w/Dark and Dragonite w/Dragon, so that's a start at least. Sadly neither of my Omastars have the useful legacy rock moveset, oh well.