Why do movies sometimes come out on Wednesdays?

LA Times elaborate it quite well with some official opinions:

"We just believe releasing it on a Wednesday before a four-day weekend is like having a rolling sneak preview." - Chris McGurk

"You get a lot of positive word of mouth going the weekend. You have two days of people validating the movie locally." - Jack Foleyinto

They also provide some examples too:

While "The Dark Knight's" summer receipts are so stunning it's not a fair predictor of any other movie's performance, the Batman sequel's midweek numbers have shown that moviegoers will flock to the multiplex on days other than the weekend.

In breaking so many box-office records (on Monday, it became the fastest film to surpass $400 million domestically, doing so in 18 days), "The Dark Knight" has sold more than $8 million in tickets every weeknight except Monday (when it grossed $6.3 million), with its earliest midweek nightly grosses topping $20 million.

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Some distributors say the summer's overall returns -- coupled with the consistent midweek revenue -- suggest that some people hurt by the struggling economy are going to the movies rather than taking off on vacations.

"It's a sign that people are not only picking movies as the weekend choice for entertainment but also in the middle of the week as well," says Jeff Blake, vice chairman of Sony Pictures Entertainment.

Sony moved up the release of "Pineapple Express" from Friday to Wednesday for several reasons: to avoid the Olympics, separate itself by a week from next Wednesday's "Tropic Thunder" (which Paramount and DreamWorks previously moved from Friday, Aug. 15, to Wednesday, Aug. 13) and generate early heat for its R-rated stoner comedy. And as they themselves said "Wednesday has become the new Friday".

One of the hidden benefits of the Wednesday premiere is specific to risqué comedies like "Pineapple Express" and "Tropic Thunder."

"The hardest genre to market is the R-rated comedy," says Paramount Vice Chairman Rob Moore, "because you can't show the movie's funniest moments on television."

Distributors say Wednesday openings -- especially late in the summer, when kids are starting to return to school -- actually boost, rather than cannibalize, weekend grosses.

"Wednesday is not a dead day at any time of the year, frankly, because of what it can do for your film," Foley says. "You get a lot more money on the Wednesday and Thursday of that first week than what you would get on the following Wednesday and Thursday. And I would rather get as much of my money up front then let the movie play out week after week."

But it is also a risky move too as mid-week review will effect weekend collection too.

And it's not limited to Hollwyood only but Bollywood also joining this trend occasionally : Is Wednesday the new Friday in Bollywood?

Studios can choose to release a movie any day of the week. However, movie attendance is a crowd phenomenon. An important part of the moviegoing experience is watching the show in a theater full of dozens, or even hundreds of people. When these crowds fail to materialize, the movie "bombs" and its run in theaters is cut short.

What we typically see with Wednesday openings is that the film is being released to capture additional "weekend-like" days associated with holidays. An example is Mary Poppins Returns, which was released on Wednesday, Dec. 19 to take advantage of a 5-day weekend. This is more often done with family-friendly movies; for example, Aquaman was released two days later (though it effectively opened that Thursday with 5pm preview shows). In the earlier case, Disney was fairly confident that Mary Poppins would perform well enough at the box office to overcome the fact that it opened on a Wednesday. However, in reality, the large crowds didn't start rolling in until Friday, and it performed very well throughout the whole holiday break.

So, why are movies released on Friday (Thursday night)? Because the buzz generated by a whole bunch of people going to the theater to see the movie draws more people to see the movie. Today, with social media, it's even more important to get as many people talking about a movie (good or bad) at once to populate the newsfeeds with reminders to go watch it. When a big movie opens on a Wednesday, it risks damaging the strength of this wave of buzz (and by extension, the box office revenue).