"All Summer in a Day" is one of Ray Bradbury's most famous stories because it touches on the theme of cruelty toward those who are different.
One of the reasons why Bradbury's science fiction is so effective is that he takes these universal stories—in this case being cruel to Margot, a little girl who had experienced the sun on earth in her past—and moves them to another planet. This story, which takes place on a constantly raining Venus, is about a group of school children who have never experienced the warmth of the sun and, on this day, they get to experience it for the briefest of times. When Margot recounts what it was like to feel the sun, the other children become angry and jealous and shove her in a closet, where she misses the cloudless and rainless moments. At the end of the story, after the sun has disappeared until who knows when, the students feel guilty about what they did to Margot and let her out of the closet.
This is a sad story because readers can see themselves in the cruel school children. How often do we shun those who might act a little differently and then metaphorically shove them into closets so they can't participate? How often does this happen to us? How often, unlike Margot, do we keep our mouths shut because what we have to say is contrary to common thought? This is the theme of this story.