Re: Harry Potter and the Half Blood Prince
The problem with this theory is that the summer is flooded with big releases, so there's really a shorter window for repeat business as opposed to the fewer big releases that the fall would have.
This is just an example of the intricacies of microeconomics. There could be doubt with a new movie entering the season with low-entry barriers and many firms with an expected/theorized audience level. However, with an established franchise movie elevating it over the homogenous products scale, the model of perfect competition is skewed or utterly erased when market power is established. Given the audience is already in place, product timing and placement becomes the key component in the capital gains equation superceding competition risks especially with the inclusion of advanced future release knowledge.
And for this reason summer movies have historically outperformed other seasonal releases (Here's some ranking with just U.S. Domestic stats).
Titanic: December 19th, 1997
Star Wars: Episode IV - A New Hope: May 25th, 1977
The Dark Knight: July 18th, 2008
Shrek 2: May 19th, 2004
E.T. the Extra-Terrestrial: June 11th, 1982
Star Wars: Episode I - The Phantom Menace: May 19th, 1999
Pirates of the Caribbean: Dead Man's Chest: July 7th, 2006
Spider-Man: May 3rd, 2002
Star Wars: Episode III - Revenge of the Sith: May 19th, 2005
The Lord Of The Rings: The Return of The King: December 17th, 2003
Spider-Man 2: June 30th, 2004
The Passion of the Christ: February 25th, 2004
Jurassic Park: June 11th, 1993
The Lord Of The Rings: The Two Towers: December 18th, 2002
Finding Nemo: May 30th, 2003
Spider-Man 3: May 4th, 2007
Forrest Gump: July 4th, 1994
The Lion King: June 15th, 1994
Shrek the Third: May 18th, 2007
Transformers: July 2nd, 2007