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?