Luc Besson insisted on keeping this date. He knew that Valerian won't get IMAX and PLF screens due to Dunkirk releasing at the same time but he stuck to his guns. He didn't budge even when it was apparent that Valerian will bomb. He could have moved the movie to August or September but nah, he doesn't care about the BO revenue. Translation: He basically shot himself in the foot with that egoistic decision.