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.
The phrase in its modern sense came about in the second half of the 20th century and became widely used toward the end of the 1980s. Many documented instances from the late ’80s and early ’90s are in rap lyrics, and many other examples from this period are in publications focused on African-American culture. Today, though, the phrase is not at all confined to those contexts.
Back in the day, it was okay if Wonder Mike liked a little too much hot butter on his breakfast toast. [Vibe (1993)]
This album has the voices of people from back in the day to now, from India all the way to America. [quote in Billboard (1994)]
Back in the day, a group like, say, Nektar would have throttled a sound as darkly beautiful as that opening for 17 stoner-worthy minutes. [Spin (1995)]
[T]hey became educators, bought their own houses — and this was in the late ’60s and ’70s. That was unheard of back in the day. [quote in Ebony (1998)]
But back in the day, 1940 symbolized only a glorious achievement, a crowning moment for “the Classiest Team in Hockey.” [The 100 Greatest Days in New York Sports, Stuart F. Miller (2006)]
But we’ve known O’Brien is a fighter since back in the day, when he was the David to Jay Leno’s Goliath. [Boston Globe]
Trevor is an inveterate drug user and low-life who worked with Michael back in the day. [Guardian Games blog (2012)]