You can find the answer to this at the very beginning of Chapter 8. In that chapter, we find out that autumn turned to winter for the first time since 1885. Obviously, winter always follows fall, so what this means is that it actually got quite cold for the first time since that year. Atticus said that the two week period was the coldest Maycomb had had in all that time.
In addition to this, it snows for the first time since 1885.
The reason this matters is because Mr. Avery says that this kind of weather is caused by disobedient kids. So Jem and Scout think it's their fault.