Source : Webster's Revised Unabridged Dictionary (1913)

Midsummer \Mid"sum`mer\, n. [AS. midsumor.]
   The middle of summer. --Shak.

   {Midsummer daisy} (Bot.), the oxeye daisy.

Source : WordNet®

midsummer
     n : June 21, when the sun is at its northernmost point [syn: {summer
         solstice}, {June 21}] [ant: {winter solstice}]