Disney tends to put their big releases Memorial Day or July if you look at their history in regard to other franchises. Pirates sequels had those dates. Prince of Persia, Narnia, Sorcerer's Apprentice, all late May or July. They like the Pixar spot right after school gets out. So I wouldn't completely rule out those dates. Mid July would fit the bill should they deviate from May. I'll be kinda surprised if they don't schedule it then, assuming Marvel intends to launch another decent sized movie earlier in the year. You aren't going to release Ant-Man in April and Avengers 2 in May for example because that will kill Ant-Man. If Avengers is the only movie schedule for 2015, then May makes perfect sense. Otherwise, you go to the 2011 schedule, with the lesser known Thor opening the season and Captain America essentially finishing it.