in one’s day—(also: in one’s days)
1. during one’s lifetime:
- And did not he [Bacon] cling to geocentricism, which was still prevailingly current in his day?
2. in a period of one’s success, prosperity, etc.:
- I know of one eminent man of science, who was a burning and shining light in his day.
on one’s day—when performing as well as one can:
- On his day he’s the finest goalkeeper in the country.