The most obvious of those is that Avatar released in December of 2009, years before studios decided on a year-round blockbuster season.
That’s something Avatar arguably started, Disney continued with Star Wars and will soon alternate holiday seasons between Star Wars movies and the Avatar sequels.
While we sing the praises of Avatar’s record, it must be said that for all the factors like inflation and theater count, James Cameron’s film did have some distinct advantages that Avengers: Endgame did not enjoy.