I think Prince of Persia was a May or June film. I clearly remember it being before July. But really, used to be June and July were the busiest months for movies... now they like move everything to April. Though, I'm sure they were expecting a LOT of 1990's gamers to actually REMEMBER Prince of Persia. Seriously, how about a movie based off of Out of this World, or Panic Restaurant, or Uforia while we're at it. As long as it's not someone people actually KNOW. Airbender... well, that's a movie that goes to show that M. Night Shabbadoo should NEVER EVER EVER be near a camera again. Even to take family photos. He'd manage to make that suck too....I remember this year, from July to August we had a flurry of films from Prince of Persia to Avatar(err I mean "The Last Airbender") to Clash of the Titans to Robin Hood that I never had any interest in. You're right, that August would be a death knell and bad sign. December is a very time honored respectable time to release.
A Muppet movie would have to compete in the overflowing bulbous gut of Summer blockbusters. I've seen huge budget superfilms fall to even bigger budget superDUPER films that were released on the same weekend, poorly timed. December is quiet enough for them not to have some Marvel comics character people only remember because there's a movie on their tail, but busy enough to get the Christmas vacation, too cold to go anywhere else, we're sick of going to the mall crowd. Anytime between that (except maybe Thanksgiving weekend) would be dangerous for this film.
Which brings me to this... remember last year when the movies weren't as plentiful due to the writer's strike years earlier? We didn't have 8 big budget bombs in one month, and movies that could get an audience thrived. All too often movies that sort of have an audience are released the same 2 week period as a movie everyone's going to see, and loses whatever potential audience it has.
That said, the last live action Asterix movie in Europe was successful, but it had reviews so horrible, it actually won the European version of the Razzie.Agreed.