Crisis on Earth-X is the upcoming 4 part crossover event between Supergirl, Arrow, The Flash and DC's Legends of Tomorrow. The crossover will air over two nights from November 27th to November 28th, 2017. The episodes involved in the crossover are:

Crisis on Earth-X, Part 1" (Supergirl)
Crisis on Earth-X, Part 2" (Arrow)
Crisis on Earth-X, Part 3" (The Flash)
Crisis on Earth-X, Part 4" (DC's Legends of Tomorrow)

DC's greatest heroes unite in the 2-night crossover event, Crisis on Earth-X, starting next Monday at 8/7c on The CW. The epic DCTV “Crisis on Earth-X” crossover event begins Monday November 27th with Supergirl and Arrow and concludes November 28th in The Flash and DC's Legends of Tomorrow.