Mr. Avery's previous reference to the Rosetta Stone had Jem and Scout convinced that they were responsible for the prematurely cold weather that hit Maycomb that autumn. When it snowed the next day, the Finch children blamed it on themselves.
Mr. Avery had said that
"it was written on the Rosetta Stone [one of mankind's earliest examples of writing] that when children disobeyed their parents, smoked cigarettes and made war on each other, the seasons would change."
Nursing guilty consciences about secretly visiting the Radley Place against Atticus' instructions--and not knowing anything about the actual Rosetta Stone--Jem and Scout figured that the bad weather was their fault. Then, when it snowed the next day, they figured Avery's prediction had come true. Sure enough, Avery visited them again.
"See what you've done?" he said. "Hasn't snowed in Maycomb since Appomattox. It's bad children like you makes the seasons change."
Jem and Scout figured that the unseasonable snow was their fault, all right, but they didn't feel too guilty since they were having such a good time playing in the snow and building a snowman.
... if this was our reward, there was somethin to be said for sin.