Midnight premieres no longer first showing of movie

TL;DR:

Studio executives allow earlier showings of films, although advanced screenings also allow select groups of people early viewings.

Long Answer:

There could be a variety of reasons for this.

Firstly, your question is specifically about The Hunger Games - Mockingjay, Pt. 1. Are you sure the listings you are seeing are just for that film? Quite a few cinemas in my area are showing the first two films back to back, with the third being shown a minute after midnight. So that's one possibility.

Another is that advanced screenings take place all around the country. This can be for critics only, so they can have a review written in advance of the movie release, so it can be printed on major websites and in major papers on the day of release. However, there are also advanced screenings that are sometimes shown at select theatres, to boost the film and attract more interest. The quicker people see the film, the more publicity it will get.

A further possibility, and this is pure guesswork on my part, is that films can be shown before their official release date if another local cinema breaks the embargo. This is guesswork as I say, but I base this on my experiences as a computer games sales assistant when I was younger. We were barred from releasing new games until a Friday morning, at one minute past midnight. But if any evidence arose that showed a local shop had broken the embargo, we could sell, sell, sell without any fear of reprisal. In fact, given how much money games brought in, several stores deliberately broken the embargo, knowing the fine wouldn't compete with the revenue they brought in.

However, the final and most likely probability is that it simply gives more people a chance to see the film earlier, rather than having to wait until midnight. SlashFilm did an article on this. To quote some select parts of it:

Midnight openings have become an important part of the moviegoing landscape in recent years, but of course not everyone can stay up until midnight, let alone stay up until midnight, sit through a two-hour movie, and drive home very late at night. So why not bump up the time just a few hours earlier to sell a few more tickets? That’s the reasoning some studios are taking, pushing their releases to Thursdays at 10 or even 7 PM.

They interviewed Paramount's Head of Distribution, Don Harris, who said:

“As we looked at the numbers of kids that were going to be out of school, we thought, “why not let theaters open it for their prime shows and then play it at 10 and again at midnight if they wanted to?’”

In a Hollywood Reporter article, Chris Aronson, Fox's president of domestic distribution was asked about his decision to release Taken at 10pm. He said:

"We wanted to find out if opening at 10 p.m. would stimulate late-night business," says Chris Aronson, Fox president of domestic distribution. "In my estimation, it did."

An anonymous executive in the article also said:

"I don't like to open a film a day early, but if you are going to get the moviegoer who may shy away from a midnight screening, then I support it," says one studio distribution executive. "This is happening because late-night business was in the toilet."

Conclusion

Ultimately, studio executives wanted it to get more people seeing the film. You're not the only one to think it devalues the midnight showings, as people on the articles I posted show, but it has improved revenue flow (apparently).

It's also possible to see advanced screenings of films, which are specifically targeted at small groups of people, before a film's official release.