Christmas features so heavily not because it was originally intended to be released in winter, but because of what the movie is about. It felt like a Christmas movie to the makers.
Jenna Anderson in her January 2019 article for ComicBook.com quotes
David F. Sandberg:
"Yeah, it's all about family and like Christmas time and this family holiday," Sandberg explained. "It's about finding your family ... with these foster kids and everything. So yeah, it's very appropriate for the story."
and from an earlier interview with Sandberg and Zachary Levi:
“It feels like a Christmas movie to me," Sandberg revealed. "It’s about family, and finding your family, you have these orphans or foster kids... and Christmas is very much a family holiday, so it sort of ties it all together in that way."
“That was a Christmas story in [the New 52 run], so I think that’s gotta play something into it,” Levi added.
“Yeah, and that was the idea Geoff Johns had for that as well, that it’s a family holiday,” Sandberg echoed.
On January 12, 2018, The Hollywood Reporter reported
The film will open April 5, 2019, the studio announced Friday. (It had previously carved out that date for an untitled DC movie.) The month is currently pretty open at the box office, with Paramount's Pet Sematary opening a week after Shazam!.