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Here's why July 4 was a box office dud

Americans were more interested in grills and fireworks over the weekend than strippers or killer robots. Movie executives can blame the calendar for the weak results this weekend. A July Fourth falling on a Saturday, while great for weekend getaways and parties, is terrible for the film industry.

"Magic Mike XXL," the followup to 2012's hugely successful Channing Tatum as a male stripper, failed to inspire—and made a lousy $12 million in its opening weekend, according to data from Rentrak. Meanwhile, "Terminator Genisys" pulled in just $29 million in its debut.

The most recent batch of flicks did so poorly that the month-old "Jurassic World" was the leading movie with $31 million over the weekend. Usually, the top Fourth of July movies are released that week.

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It turns out that the day of the week that July 4 falls on can have a huge impact on how movies perform at the box office around the holiday. CNBC analyzed box office performance data from Box Office Mojo for the week of July 4 over the past 13 years.

"Most people are going to barbecues and fireworks displays," said Bruce Nash, founder of Nash Information Services, which tracks the film industry. "So it tends to be very quiet for movies."

When July 4 falls on a Saturday—normally the busiest day for moviegoers—movies make an average of $20.8 million less than that same Saturday in years when July 4 falls on a different day. Friday Fourths of July also underperform by 34 percent. So for movie studios, the worst possible day for a floating holiday to fall is Saturday, as it did this past weekend.

When July 4 is during the week, the movie industry enjoys a bump as people flock to the box office on July 3, in anticipation of the following day off of work. And when the holiday is during the week, there's the normal weekend bump the movie studios can count on.

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One example of a studio taking advantage of the calendar, Nash said, was in 2012 when "The Amazing Spider-Man" came out on Tuesday, July 3. "What Sony tried to do was give Spider-Man a six-day opening weekend," Nash said. "It worked for that film in that they made $137 million by the end of Sunday."