The day referenced by "back in the day" can be anywhere from a few months ago to many years. It's more about the difference in attitude than in time.
Back in the day, we partied more. We stayed out later. We drank in excess. Back in the day, we hated the man and rebelled against our parents. We wanted to try every high there was; chemical or emotional. We had no responsibilites, no laws, and no shame. Back in the day, we loved everything and feared nothing.
You can't ever go back to the day, but you can always move forward to other days.