You can already buy tickets for as many of the movies as you like. The first five days of the festival are focused on showing the movies in order (important if you're new to the MCU). After that, however, it gets interesting: the next two days are themed around origin stories and alliances, while the last day will focus on two titles chosen by a vote that runs until August 17th. If you miss one of the IMAX screenings, there's a real chance you can catch the relevant movie again before the festival is over.