Quick summary on the event in the square.

Are you referring to the day of the trial? 

In chapter 15, i think so

In Chapter 15, Atticus is guarding Tom Robinson in his cell at night.  A shadowy group of men emerges. Atticus informs them that the sheriff is nearby, but they counter that they called him into the woods on false pretenses. Atticus still seems unperturbed.The men basically threaten Atticus if he doesn't hand over Tom to them. Scout and Jem both run into the square. Jem refuses to go home while his father is in danger. Scout discharges the situation by appealing to Mr. Cunningham's humanity.