What "disaster" happened at Christmas between Scout and Francis, in the book, To Kill a Mockingbird?
The fight that occurs is indeed disastrous, but I think what's more is that this reveals for the first time that when the going gets tough for Atticus, he may struggle to have the support of his extended family.
Scout doesn't understand why Francis would be so spiteful as to say something that hurtful. Her only reaction is to deal with it physically.
On one level, this foreshadows the trial, but beyond that it foreshadows how Alexandra is going to deal with Atticus' moral fortitude, and the kids are going go have to watch.
Francis is one of Scout's cousins. Scout and Jem and Attticus go to Aunt Alexandra's place for Christmas and Francis is there because Aunt Alexandra is his grandmother.
Scout and Francis end up fighting, or rather Scout beats Francis up. This is because he tells her all kinds of bad stuff that Aunt Alexandra says about Atticus and her kids. He says that Aunt Alexandra says that Jem and Scout run too wild and that Atticus is a nigger lover.
Scout gets in a lot of trouble with Uncle Jack because of this, but at least he does not tell Atticus what she has done.
Yes, Scout fights her cousin Francis for saying the ugly things he says, but I think the issue is bigger than just the fight. We all know that Scout is quick to fight and think later, but this time it is with a family member. Francis doesn't mind telling Scout exactly what Aunt Alexandra has said about Atticus and the way he is raising his children. He even makes fun of Dill, and this sets Scout on edge.
"If Uncle Atticus lets you run around with stray dogs, that's his own business, like Grandma says, so it ain't your fault. I guess it ain't your fault if Uncle Atticus is a nigger-lever besides, but I'm here to tell you it certainly does mortify the rest of the family-"
"Francis, what the hell do you mean?"
"Just what I said. Grandma says it's bad enough he lets you all run wild, but new he's turned out a nigger-lover we'll never be able to walk the streets of Maycomb agin. He's ruinin' the family name, that's what he's doin."
After Francis tells Scout just what the family thinks of Atticus, Scout fights him. In her young age, she doesn't understand the ramifications of what has been told to her. Francis is just repeating what his grandmother has said to him. This is the first time Scout is hearing these horrible things being said about her father by his own family. It is a shock to her, and she has a hard time understanding it. This whole event takes place to set up the way the town is going to feel about Atticus and the things Scout is going to hear.