The Collaborative International Dictionary of English v.0.48:

Summery \Sum"mer*y\, a.
   Of or pertaining to summer; like summer; as, a summery day.
   [1913 Webster]

WordNet (r) 3.0 (2006):

summery
    adj 1: belonging to or characteristic of or occurring in summer;
           "summery weather"; "summery dresses" [ant: autumnal,
           vernal, wintery, wintry]

Moby Thesaurus II by Grady Ward, 1.0:

86 Moby Thesaurus words for "summery":
   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, 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