The Collaborative International Dictionary of English v.0.48:

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]

The Collaborative International Dictionary of English v.0.48:

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]

WordNet (r) 3.0 (2006):

heyday
    n 1: the period of greatest prosperity or productivity [syn:
         flower, prime, peak, heyday, bloom, blossom,
         efflorescence, flush]