It's the same question every Friday afternoon.
"What am I doing this morning?(tomorrow morning)
"You have a free morning. We don't go anywhere. We'll all stay together here."
"You can sleep in."
"What is sleeping in?"
"You know how we have to get up early usually so we are not late for preschool? Well on Saturdays there is no time to get up. You can stay in bed as long as you like."
"Oh. Okay, I am going to sleep in."
I really wish I had explained the meaning of sleeping in before this.