Somebody's heyday is the period when they were at their best. The years everything worked.
The word always looks back, and it usually carries a note of decline, since saying that a place is past its heyday means it has fallen off. It suits people, towns, industries and bands equally: in his heyday, the heyday of the railways, the heyday of the seaside resort. It takes of or a possessive. The word has nothing to do with hay, and comes from an old exclamation of excitement.
1nounheydayThe period of greatest success or vigour.
“In its heyday the mill employed two thousand people.”
Late 16th century, from earlier heyda (1520s), as exclamation; compare hey, hei and Swedish hej då. Sense “period of success, vigor” is a as heyday based on unrelated day (as “period of time”) – compare day in the sun.
Etymology from Wiktionary, CC BY-SA