Back in the days
Definition 1 of 1
Pro Tip 1/3Nostalgic Tone
The phrase often implies a fond reminiscence of the past. 
Back in the days, we would spend all summer outdoors.
Pro Tip 2/3Not Precise
The time period referred to is not specific and can vary greatly. 
Back in the days, people wrote letters instead of emails.
Pro Tip 3/3Colloquial Usage
It's casual language, used more in spoken than in written English. 
We used to go to the arcade back in the days.