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]

Moby Thesaurus II by Grady Ward, 1.0:

86 Moby Thesaurus words for "aestival":
   arctic, autumn, autumnal, beryl-green, berylline, blood-hot,
   blood-warm, blue-green, bluish-green, boreal, brumal, calid,
   canicular, chartreuse, chloranemic, chlorine, chlorotic, citrine,
   citrinous, emerald, equatorial, equinoctial, foliaged, genial,
   glaucescent, glaucous, glaucous-green, grassy, green,
   green as grass, green-blue, greenish, greenish-blue,
   greenish-yellow, greensick, hibernal, hiemal, holly, ivy,
   ivy-green, leafy, leaved, luke, lukewarm, midsummer, midwinter,
   mild, olivaceous, olive, olive-green, out of season, porraceous,
   room-temperature, seasonal, smaragdine, solstitial, spring,
   springlike, subtropical, summer, summerlike, summerly, summery,
   sunny, sunshiny, temperate, tepid, thermal, thermic, toasty,
   tropical, unfrozen, verdant, verdurous, vernal, vernant, vert,
   virescent, warm, warm as toast, warmish, winter, winterlike,
   wintery, wintry, yellowish-green