Complex sentence: The crossover began on December 1,2015 with "The Flash" episode "Legends of Today" and concluded on "Arrow" on December 2 with the episode "Legends of Yesterday".
Split into simpler sentences: The crossover began on December 1 with "The Flash" and concluded on the "Arrow" on December 2. The first part was titled "Legends of Today" and the second part "Legends of Yesterday".