The week before Thor makes a lot of sense (likely on some random ep of Jimmy Kimmel, like with Spiderman), but I could also see them holding it for The Last Jedi. Infinity War doesn't need a huge promo cycle - the pitch sells itself. Give the world footage of Starlord & Spiderman high fiving, and they'll start spamming Fandango for release day tickets. It's just a matter of getting that message out to as many people as possible. Launching that trailer with Star Wars creates a huge first impression.