Back in the day
Definition 1 of 1
Pro Tip 1/3Nostalgic Undertone
Implies a fond or wistful memory of earlier times, often with a hint of longing. 
Back in the day, summers seemed to last forever.
Pro Tip 2/3Not Literal
It doesn't specify an exact time. It's a general reference to the past. 
Back in the day, high school felt like a different world.
Pro Tip 3/3Conversational Use
Most common in informal talks among friends, family, or peers. 
Back in the day, we didn't have smartphones to keep us busy.