Over the last few years, the CW has been growing its stable of DC shows and reminding us of that with a big, juicy crossover right around mid-season. The crossover is happening again this year, but the network is mixing things up to properly include all four superhero shows and make the event a bit easier to digest.
Last year, the event included Supergirl, but really started on Tuesday with The Flash. It went through Thursday, running through Arrow and DC’s Legends of Tomorrow. This year, Supergirl will kick off the event, which will include all four shows. Instead of going across the whole week, though, the network is shifting things around and turning it into a two-night affair, with two episodes each night.
Article source: https://www.technobuffalo.com/2017/08/04/cw-dc-crossover-supergirl-arrow-flash-legends-tomorrow/