It was originally going to open next month, but was moved to Feb. 14. Now, it will open June 7, as the summer is always a good and lucrative time for superhero movies, and that was pretty much the impetus for the change.
Speaking to Collider, Dark Phoenix's director, Simon Kinberg, explained that it all came down to a combination of special effects work, the chance to make more money, and the oppurtunity to bring in more viewers, especially ones from China, where the X-Men movies are quite the hit.
“Some of the trailers had ‘Coming Soon’ on them and some of them said ‘February 14th’. It was a date, February 14th, that we initially liked for the movie but when we realized that we weren’t gonna have the visual effects ready to release it globally the way we wanted to, and the Gambit date opened up because it wasn’t gonna be ready for that June 7th date," he said. "We looked at that date versus the February 14th date, the studio did and we did, we felt like that June date was a bigger opportunity for us globally. More screens, more IMAX screens, a better chance to play in China where these movies have a massive following. What we found when we released the trailer was that 44, 45 million views of the trailer were in China alone… Specific for us, it is an opportunity to be a bigger movie day-and-date globally.”
In August, production entered the reshoot/pickup phase in Canada, with some outlets reporting that the film needed an extra three months of filming. This turned out to be false; it was only two-and-a-half weeks long. According to Kinberg, the reshoots mainly focused on improving the story's third act.
“Mostly we focused on the third act, both emotionally and in terms of the physical scale and the action of the third act that we adjusted," he said. "And then there were certain things in the first two acts of the movie — very few things, but to set up those changes in the third act we had to adjust some things in the first two acts so that that all was fluid and felt consistent.”