People often have fond memories of the past. When you're with old friends and talking about good memories from the past, you can express your feeling of nostalgia by saying:
Those were the days.
This means "That was a really great time in our lives."
A similar expression is "the good old days".:
Those were the good old days.
If you want to talk about a period that was difficult or negative, use "tough times":
Those were tough times, weren't they?