As with most of Bradbury's works, the symbolism can be tied back to man's basic quest to figure out who he is and how to survive. Often the sun also symbolizes a promise of tomorrow or immortality in Bradbury's writings.
In "All Summer in a Day" the sun symbolizes a reminder of who they all really are as humans. It symbolizes emotion, color, life and hope or the promise of tomorrow. The kids that lock Margot in the closet prior to the sun coming out seem incapable of empathy and devoid of any emotion that might represent the cruelty of their actions. Additionally, they are pale and colorless both in appearance and emotion. Only after experiencing the sun do they appear as though they might feel remorse for their actions and be able to express appropriate emotion. Once running into the sun the author writes:
"And the jungle burned with sunlight as the children, released from their spell, rushed out, yelling into the springtime."
The word "spell" indicates that on Venus, no one is who they really are as humans. However, when released to run freely in the sun, they are released from the spell and are described out in the sun by Bradbury in words that describe normal, human children experiencing freedom and play.
Therefore, the sun symbolizes a promise of a return to who humans really are and how they experience life on Earth.