Source : Webster's Revised Unabridged Dictionary (1913)

AEstival \[AE]s"ti*val\, a. [L. aestivalis, aestivus, fr. aestas
   summer.]
   Of or belonging to the summer; as, [ae]stival diseases.
   [Spelt also {estival}.]

Source : WordNet®

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