When did Scout and Jem fight ?

I believe you are referring to Jem and Scout's brawl in Chapter 14. This chapter starts out with Scout asking Atticus what rape means. When it emerges that Scout had already asked Calpurnia for an explanation prior to approaching her father, Aunt Alexandra is less than pleased. She is even less pleased when she discovers that Jem and Scout have attended church with Calpurnia. In a testy discussion which foreshadows Jem and Scout's physical tussle, Aunt Alexandra warns Atticus that Calpurnia would prove a bad influence on Scout in the long run. Atticus stubbornly defends his housekeeper and refuses to release Calpurnia from the household.

He reasons that Calpurnia has done well in raising his children and asserts that his children love and need her too much for him to let Calpurnia go. The tension is high as the argument continues, so Jem calls Scout away to his bedroom in order to give the adults some space. He tells Scout not to antagonize their aunt; here, he is referring to the episode in the living room when Scout briefly challenges her aunt's refusal to let her and her brother attend church with Calpurnia on a future Sunday.

At Jem's words, Scout flies into a rage. Due to her overwrought emotions, she fails to recognize the evident concern in Jem's request. Jem, for his part, is worried about Atticus' ability to hold up in the Tom Robinson trial. To Scout, Jem's maddening tendency to classify himself with the adults while relegating Scout to the status of one who needs to receive 'edification and instruction,' upsets her. The last straw comes when Jem proclaims that he will spank her if she continues to antagonize Aunt Alexandra.

At this, Scout flies at Jem in a rage.

He was still on the bed and I couldn’t get a firm stance, so I threw myself at him as hard as I could, hitting, pulling, pinching, gouging. What had begun as a fist-fight became a brawl. We were still struggling when Atticus separated us.

As punishment, Atticus sends both his children to bed. However, it doesn't take long before both recover their good graces in time to welcome the inimitable Dill into their home.