The Collaborative International Dictionary of English v.0.48:
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.] [1913 Webster]WordNet (r) 3.0 (2006):
aestival adj 1: (rare) of or occurring in summer; "the sky was a burnished aestival blue"; "estival winds" [syn: aestival, estival]