What important event does Ponyboy relate to Cherry in the popcorn line?

While standing in the popcorn line at the drive-in movies with Cherry, Ponyboy retells the story of when Johnny was badly beaten by a group of Socs. Pony says that it was a warm, spring day about four months ago, when him, Steve, and Sodapop were walking home from the DX station. As the boys were walking past the open field by the corner of the block where they play football, Steve noticed Johnny's blue-jean jacket. Steve saw blood stains on the collar of the jacket and the ground leading to the far end of the field. The boys heard Johnny moan, and when they turned his body right-side-up, they saw that Johnny had been badly beaten. Ponyboy says he was cut, bruised, and swollen all over his face. Johnny told the gang that a blue Mustang full of Socs jumped him, and one of the boys was wearing a handful of rings. Pony tells Cherry that Johnny became a nervous wreck after that night and was jumpier than ever.