The Collaborative International Dictionary of English v.0.48:

Unseasonable \Un*sea"son*a*ble\, a.
   Not seasonable; being, done, or occurring out of the proper
   season; ill-timed; untimely; too early or too late; as, he
   called at an unseasonable hour; unseasonable advice;
   unseasonable frosts; unseasonable food. --
   Un*sea"son*a*ble*ness, n. -- Un*sea"son*a*bly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unseasonably
    adv 1: not in accordance with the season; "it was unseasonably
           cold" [ant: seasonably]