In simplest terms, theme can be defined as the main message that the author is trying to get across. It is the main idea with a message. Friends might be a main idea but "friends always stick together" might be a theme. Mood can be defined as words that describe the way the story makes the reader feel or an attitude it projects. The mood of a story might be humorous, terrifying or triumphant.
The theme of "All Summer in a Day" might be described as isolation. This theme is portrayed in the character of Margot, who is not only an outcast child tucked away on the planet Venus but also an outcast to her peers. The other children with her on Venus also portray isolation, tucked away on a dark, rainy planet far from Earth and family. The fact that the author never explains why they are there, adds to the theme of isolation. The setting itself with the dark, dreary endless, rainy days helps the reader feel the theme in the form of a mood of loneliness and sadness. While the author writes briefly that the children are happy in the short time of sun, the overwhelming majority of the story's theme of isolation creates a mood of loneliness and sadness in the reader.
There are several themes to the story "All Summer in a Day". The most dominant theme can be identified as jealousy. The jealousy of the kids is what drives them to lynch Margot and deny her what she waited 7 years to see. This darker theme complements the cold mood of the story quite well. The mood of the story is generally dark and depressing. It brightens up a little when Margot describes the sun and her time on Earth, but fast drops back when the rest of the kids immediately tease her and call her a liar.