It is Hevensday today...
Fantasy and science fiction are characterized, more than by anything else, by taking place in worlds that are significantly different from the world that the writer (and reader) lives in.
The characters who live in such a world need to make choices; that's how personality is shown. That's what personality is: The values, morals, preferences and inclinations, of an individual, as seen in the choices that he or she makes.
The reader must be able to appreciate and evaluate these choices, form an opinion of them, and one criterion for good fantasy or good science fiction is that the choices must be dependent upon at least some of the ways in which the world of the story is different from the world that the writer (and reader) lives in1. In order to make this evaluation of the character, the reader must therefore know how the world works, how the new or strange or otherwise different things in the world works. The reader has to know the rules.
How can the reader come to know the rules?