From Longman Dictionary of Contemporary Englishsomebody’s/something’s daysomebody’s/something’s dayTHENa successful period of time in someone’s life or in something’s existence My uncle was a famous radio personality in his day (=at the time he was most successful). Don’t be too disappointed you didn’t win – your day will come (=you will be successful in the future). Game shows like that have had their day (=were successful in the past, but are not anymore). → day
Examples from the Corpus
in his day• The vocals are strong, clear and smooth, crooning with the memory of what Sinatra was back in his day.• And third, Marx was single-minded and relentlessly determined to bring about change in his day.• He was not naive, nor was he blind to the greedy practices of some merchants and manufacturers in his day.• He must have enjoyed it when he was scoring off his pupils in his days as a college professor.• The man had been a famous marathon runner in his day.• They say in his day he was a great one for the ladies.• Wagner in his day had preached purity of heart and heroic deeds. 