26/3/19 Back in the day

Back in the day is an American idiom used to refer to an earlier time, especially one the speaker remembers fondly. Unlike similar phrases that state a more specific time—e.g., back in the days of dial-up internet, back when we were young—back in the day is not part of a larger phrase and doesn’t specify the time it refers to; day is not qualified. We must use context to infer what time the speaker is talking about, so it’s similar to phrases like a long time ago and some time ago.
 
Back in the day, it was okay if Wonder Mike liked a little too much hot butter on his breakfast toast.