Beverly Kranz of Collectors Ink in Chico, California saw our article on the retailer poll on the date for Free Comic Book Day and feels that consistency is important (see 'Retailers -- Vote on Free Comic Book Day'):

Ultimately the success of Free Comic Book Day depends on having people show up at your store and participate.  Therefore, the consistency of having FCBD the same weekend every year will make it easier for people to remember to come, and also for FCBD to be perceived as an actual annual event and not just as a promotion for a movie (whose release date can change at any time, by the way).

Also, even though there are a lot of movies in production so that it looks like this won't happen, anchoring FCBD to the movie of the year can be a real problem if we hit a year where there is no big movie to tie it to.