Source : Webster's Revised Unabridged Dictionary (1913)

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.

Heyday \Hey"day`\, n. [Prob. for. high day. See High, and
   {Day}.]
   The time of triumph and exultation; hence, joy, high spirits,
   frolicsomeness; wildness.

         The heyday in the blood is tame.         --Shak.

         In the heyday of their victories.        --J. H.
                                                  Newman.

Source : WordNet®

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