solstitial

from The Collaborative International Dictionary of English v.0.48

Solstitial \Sol*sti"tial\, a. [L. solstitialis: cf. F.
   solsticial.]
   1. Of or pertaining to a solstice.
      [1913 Webster]

   2. Happening at a solstice; esp. (with reference to the
      northern hemisphere), happening at the summer solstice, or
      midsummer. "Solstitial summer's heat." --Milton.
      [1913 Webster]