Source : Webster's Revised Unabridged Dictionary (1913)

Estival \Es"ti*val\, a., Estivate \Es"ti*vate\, v. i.,
Estivation \Es`ti*va"tion\, n.
   Same as {[AE]stival}, {[AE]stivate}, etc.

Source : WordNet®

estival
     adj : (rare) of or occurring in summer; "the sky was a burnished
           aestival blue"; "estival winds" [syn: {aestival}]