in (one's) day

When one was younger; in one's younger days. In my grandparents' day, planning a wedding wasn't such a colossal ordeal. Back in my day, the only thing we had to do for fun was stickball, and we were grateful for it! In my day, school was actually affordable. Now, high school can cost tens of thousands of dollars—a year!

in somebody’s day/time

1 when somebody was most successful, famous, etc: He had, in his day, been one of the greatest opera singers in the world.
2 at the time when somebody was alive; when somebody was young: In my grandmother’s time, women were expected to stay at home and look after the children. ♢ In my day, nobody would have spoken to the boss like that.