I've followed movies and movie discussions over the last decade. This is the conventional wisdom:

January is the dump month. If it's released in January, it is either junk or an experimental movie looking for a quick look (e.g. Cloverfield).

February is another dump month, though these movies may have a slightly higher hope. Jan-Feb is Oscar season. Small Oscar contenders are generally given wider releases during these two months. With the younger audiences in school and winter weather a major hazard, it is difficult to create that blockbuster that will draw audiences day after day.

March and April are the wild card months. Spring break, Easter, and better weather means that if a must-see movie is released, it will be seen by those that must see it. In the past, May was also included, but that changed about ten years ago.

May is the early blockbuster month. Generally, it is either the first week or the Memorial Day (U.S.) weekend. Since the high budget blockbusters are few and far between (Matrix was a March release), the first weekend in May often has (over the last decade) been a single blockbuster followed by more big movies the week before and the week of Memorial Day.

June is the normal summer release. The young people are out of school and can see a movie any day of the week. The blockbuster weekend can be the blockbuster week. These movies are not the biggest of the summer, but they are big movies.

The 4th of July (in the U.S.) is the BIG blockbuster weekend. Many people take 3 or 4 days off over that period. The rest of July follows similarly to June.

The first week in August is the last chance for any blockbuster. With most schools in session starting in the middle of the month, weekday attendance starts to drop off. To make the big money, a movie must be released in the first weekend. A movie released after that (especially if it is MOVED) is considered dumped.

September and October is a continuation of late August for any blockbusters. The best releases now are the small releases. Oscar contenders often try to get the jump by getting released now. A slowly expanding release is the norm.

November and December are the Oscar/blockbuster season. Movies that have a chance at getting nominated are best released here. The last week in November and the last two weeks in December are mini-summer type seasons.

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Keeper of the Jabberwock