What exactly is "Your day"?

You've heard it before, in my day this, in my day that, but is there a certain age range, when you were young/in your prime/heyday it applies to? Maybe 18-40 or can it just mean a time period you liked.the most in your life Maybe it varies by generation. For the greatest generation the 40's to the 60's, for the baby boomers 60's to 80's, etc.

Lynnmarie2019-12-22T04:37:48Z

I think it refers to the time you are 18-40ish. 

Pearl L2019-12-22T02:22:05Z

the day something good happened