The day of the week is not directly stated in S.E. Hinton's novel The Outsiders, but enough clues are given to make a reasonable inference. 

The book opens with Ponyboy walking home from the movies. He is attacked by the Socs, and no time frame is given for when this takes place. Darry rescues him along with his other brother Sodapop and other members of their gang. In the following pages, the narrator of the book, Ponyboy, describes each of his friends and gives information about their background. 

On page fourteen, Dallas Winston, known as Dally, announces that he's going to the drive-in movies the following night, and wants to know if anyone else wants to go. No day is given, but since Darry allows Ponyboy to go because "it ain't a school night," Dally must be planning to go to the movies on either a Friday or Saturday. The night they go to the drive-in is the same night that there is a fight between Johnny and Ponyboy and the Socs. The fight occurs because Johnny and Ponyboy have been socializing with two Soc girls, Cherry and Marcia. Two-bit was with them, as well, but he left before the fight broke out. Cherry is Bob's girlfriend. Bob is a Soc.

The Socs are drunk when they attack Ponyboy and Johnny, and they force Ponyboy's head into the fountain in the park. Johnny believes they will drown Ponyboy, and kills Bob with his switchblade. After the murder, Johhny and Ponyboy find Dally for advice on what to do next. Dally gives them a gun and fifty dollars and tells them to hop a train to Windrixville, a neighboring town where there is an abandoned church they can stay in. 

Five days later, Dally comes to check on them. 

On the fifth day, I had read up to Sherman's siege on Atlanta in Gone with the Wind, owed Johnny a hundred and fifty bucks from poker games, smoked two packs of Camels, and, as Johnny predicted, gotten sick. I hadn't eaten anything all day, and smoking on an empty stomach doesn't make you feel real great. I curled up in a corner to sleep off the smoke. I was just about asleep when I heard, as if from a great distance, a long low whistle that went off into a sudden high note.

The whistler is Dally, who comes to update them about what is going on. He takes them to Dairy Queen for some food, and when they return to Jay Mountain, the church is on fire. 

If they went to the movies on a Friday night, they would have saved the children from the fire on a Tuesday. If it was a Saturday night, they would have saved the children on a Wednesday.