Complex sentence: The crossover began on December 2,2014 with "The Flash" episode "Flash vs. Arrow" and concluded on the "Arrow" on December 3 with the episode "The Brave and The Bold".
Split into simpler sentences: The crossover began on December 2 with "The Flash" and concluded on the "Arrow" on December 3. The first part was titled "Flash vs. Arrow" and the second part "The Brave and The Bold".