This is derived from a reference to the days when it was fun and socially acceptable to sit around the cafeteria table and discuss various alternate meanings for the term applebutter.
"Yeah, we used to do that stuff all the time. Those were the applebutter days."
"Back in the applebutter days, cartoons didn't have to be politically correct."
"I don't want to go to work today. I want to go back to the applebutter days of college."