Those were the days
Definition 1 of 1
Pro Tip 1/3Nostalgic Emotion
The phrase often express a longing or affection for the past. 
Listening to these songs, those were the days, my friend.
Pro Tip 2/3Positive Reminiscing
It's used to recall positive and enjoyable times, not sad or negative ones. 
When we gathered for family picnics, those were the days.
Pro Tip 3/3Colloquial Use
Mainly used in informal conversations among friends or family. 
We would fish at the lake every summer, ah, those were the days!