Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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