The festival takes place for about two weeks in mid-December. Check the Internet for the exact dates. The festival features mainly Latin American films, often those you'd never see in the United States. However, there are also European and American films. Directors, actors, and other artists come from all over to attend. In 2002, Gabriel Garcia Marquez, Matt Dillon, and the directors of Cidade de Deus and El Crimen del Padre Amaro attended. The festival gives more energy to the city than any other time of year. Also, as the Havana arts scene and Cuban culture lack the pretensions that seem to dominate many artistic gatherings, the atmosphere is relaxed and focused on the films.
Tickets can be purchased for 40 dollars at Hotel Nacional for entrance to all films in the festival (I would say there are about six films/theater/day playing in about 20 theaters. This includes tickets to the opening ceremony and Clausura and the party afterwards.) Last year, los Van Van, Cuban's premier salsa group, played the party. And you can attend all events in your flip-flops. If you just want to attend a few films, the price per ticket is 2 pesos (about US$.08). However, you will have to line up sometimes hours in advance(the pass allows you to bypass the line). You can sneak into the opening and closing parties if you are persistent enough, or take someone's invite that they are not using.
The other great thing about the film festival is that Cubans and foreigners interact with each other. The divide which seems to exist in all but the upper circles of Cuban society disappears. Everyone is allowed and can afford to participate. If you can go to Havana in December, do it. And the weather is pleasantly brisk. For Latin jazz lovers, the Jazz Festival overlaps with the end of the film festival.