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.
Also listed in same link:
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?