From WordNet (r) 2.0 (August 2003) [wn]:
heyday
n : the period of greatest prosperity or productivity [syn:
flower, prime, peak, bloom, blossom,
efflorescence, flush]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Heyday \Hey"day`\, interj. [Cf. G. heida, or hei da, D. hei
daar. Cf. Hey, and There.]
An expression of frolic and exultation, and sometimes of
wonder. --B. Jonson.
[1913 Webster]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Heyday \Hey"day`\, n. [Prob. for. high day. See High, and
Day.]
The time of triumph and exultation; hence, joy, high spirits,
frolicsomeness; wildness.
[1913 Webster]
The heyday in the blood is tame. --Shak.
[1913 Webster]
In the heyday of their victories. --J. H.
Newman.
[1913 Webster]