While February is no longer a stranger to comic book blockbusters in the modern era, thanks to the aforementioned Deadpool and the recent, completely massive success of Black Panther, this release makes a lot more sense for the film.
But then, just two days after the first trailer was released in late September 2018, Fox did more shuffling and set Dark Phoenix down on June 7, 2019.
Rumors on the date change have suggested that the studio believes X-Men: Dark Phoenix will simply perform better then, especially in China, which went crazy for the trailer after it came out.
With the exception of Deadpool and Logan, which hit theaters in the first quarters of their respective years, the X-Men movies have traditionally been spring and summer releases, which makes this new strategy fall more in line with what we generally see.