A little backstory regarding this visual novel:
Ryukishi07 is a lovely author, and often organize and/or participate of annual fan meet-ups. During these events (the ones I'll be referring to are called "07th Party"), fans can talk with him and ask him questions, sell their 07th Expansion related doujin works, have fun with some of the activities they may want to participate during the event, cosplay, and, by the end of the event, be part of a group photo with Ryukishi, some of which you likely saw floating around Twitter and /jp/ whenever this kind of event is brought up.
However, another thing only a couple of people know about is that, he also made for the fans who came to some of these events (namely "07th Party 2" and "07th Party 3") an exclusive crossover visual novel quite similar to what Carnival Phantasm is to TYPE-MOON works, only that it's made with 07th Expansion works released at the time (which would be "Higurashi no Naku Koro ni", "Umineko no Naku Koro ni", "Higanbana no Saku Yoru ni" and "Rose Guns Days").
Thus what bring us to this release:
07th Theater is a visual novel that compiles both of these exclusive crossover chapters released during the events.
IT CONTAINS SPOILERS TO EVERY SINGLE ONE OF THESE TITLES, SO MAKE SURE TO READ ALL OF THEM BEFORE PLAYING THIS.