... in (或on)one's day (在某人)当年,盛时 in one's early days在年轻的时候;在初期 in (the) days gone by 从前 ...
原声例句
1
The family left Russia in 1919. So, the revolution, I think, starts in 1917. And things are quite complex in those early days, so there's more than one anti-czarist factor.