Of somebody’s day
Definition 1 of 3
Pro Tip 1/3Historical Context
Use it to reference someone's prime within their historical context or era. 
The author was a celebrated thinker of his day.
Pro Tip 2/3Comparative Excellence
Implies comparatively the best among peers during a past time period. 
She was the voice of her day, unrivaled in opera.
Pro Tip 3/3Nostalgic Tone
Often used with a nostalgic tone, looking back at someone's peak moments. 
He was the fastest sprinter of his day.