1.
during the part of sb's life when they were most successful, famous, etc. :
She was a great dancer in her day.
2.
when sb was young :
In my day, there were plenty of jobs when you left school.
IN SB'S DAY
Meaning of IN SB'S DAY in English
Oxford Advanced Learner's English Dictionary. Оксфордский английский словарь для изучающик язык на продвинутом уровне. 2005