"All Summer in a Day" imagines a Venus whose weather is one of almost never-ending rain, where the sun comes out for only a brief span, during one day every seven years.

Margot herself had come to Venus five years earlier, so unlike the other students in her class, she actually has memories of sunlight, thanks to her time on Earth.

The key to understanding this question is to recognize that Margot does not want to live on Venus. She is profoundly homesick and wants to return to Earth. This brings us to the image of her reaction to the shower, which is closely tied into her suffering on Venus. What defines life on Venus is its incessant rain, to such a degree that it blots out the sun, and this is one of the factors which, for Margot, makes life so miserable. Her reaction in this scene amounts to a rejection of her life on Venus, pointing back to her profound desire to return to Earth.

In Bradbury's story, Margot is the only child her age on Venus who consciously remembers sunlight. The other children of her age group came to Venus when they were so young that all they remember is the constant rainfall that continues for years and years on end. To them, all this water is normal. Margot, however, remembering sunshine, finds the rain depressing, to the point that her parents are considering returning to Earth because of her despondent state. The shower only serves to remind Margot of the endless rain, which she is thoroughly tired of. She wants to be dry, not wet, baking in the sun, not under the tyranny of a constant downpour, and so rebels against the shower. To her, the shower symbolizes the constant rain she hates.