There are times when a movie brings you face to face with the ineffable mysteries of life, when the cocktail of dramatic, visual, and auditory arts remind you of the power of art to stir your soul.
Summer is not one of those times.
Sometimes a movie is good for a quick slam bam thank you ma'am two hours of sound and fury wrapped up in eye candy, and who cares about Oscar? This is pure commerce, baby.
That kind of movie? Odds are you'll see it in May, June, July, or August.
Hollywood typically earns 40 percent of its box office revenue with domestic releases between Memorial Day and Labor Day weekends (last weekend of the month of May to the first weekend in September) when kids, especially teenagers with money and time on their hands, are out of school and can go to the movies. Hollywood studios count on the young age of moviegoers during this time, and gear their releases accordingly.
In 1999, playwright and filmmaker David Mamet elaborated on the anthropological context of these summer releases, where visual storytelling is replaced with visual spectacle:
We need not decry the summer film's vapidity. It would be inappropriate to criticize the pie-eating contest for lack of a reasonable respect for nutrition. In the summer film, drama would be as out of place as landscape design in the state fair's midway. The screenplay bears the same relation to the drama that the bumf on the cereal box bears to literature. Its writing and its production are obeisance to the god of commerce. The public pays its fine and spends its two hours in a celebration of waste in the time of abundance, the unambiguous enjoyment of the sun, the solstice festival, when worship of the antic god is all joy, and Nemesis is, for the moment, powerless. In this druidical observance, she is, in fact, ritualistically murdered--- the hero slays her at the conclusion of the summer film, and we go on our way, out into the friendly summer night.
Although today's audiences understand summer movies to be visceral thrill rides and showcases for the top action and comic stars, this was not always the case. Historically, summer was anathema to Hollywood studios. People stayed away from the theaters in the summer, some theatre even shut down-- mostly, they were not air-conditioned. But a perfect storm of factors (including the mainstreaming of air-conditioning in theatres) started a frenzy of summer releases following the success of Steven Spielberg's Jaws, released in1975. Using Jaws as their exemplar, Hollywood changed the way they made and released movies, and suddenly the studio blockbuster, infused with cash in both its marketing and its visual effect department, was paired with the summer months and cast with younger, hipper actors.
A summer theatrical release also allows for a Christmas season home video or DVD release. However, the importance of the summer movie as an economic engine is fading, as DVD sales now make up the bulk of movie studio revenue (upwards of 60%), with box office a mere 20%. Still, if a studio is going to gamble on a $100 million+ production to bring in $400 million in box office, odds are it will be released in the summer.
Sources:
David Mamet: "The Screenplay and the State Fair," in Zoetrope, Vol. 3 No. 1
Terrence Rafferty, "SUMMER FILMS: ON THE BEACH; The Movie That Created the 'Summer Movie'" The New York Times. April 30, 2000.
Dana Reinhart, et al. "Jaws: The Monster that Ate Hollywood." Frontline. November 22, 2001. <http://www-c.pbs.org/wgbh/pages/frontline/shows/hollywood/business/jaws.html> (December 1, 2006) | 
