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):

unseasonable
    adj 1: not in keeping with (and usually undesirable for) the
           season; "a sudden unseasonable blizzard"; "unseasonable
           bright blue weather in November" [ant: seasonable]
    2: badly timed; "an ill-timed intervention"; "you think my
       intrusion unseasonable"; "an untimely remark"; "it was the
       wrong moment for a joke" [syn: ill-timed, unseasonable,
       untimely, wrong]

Moby Thesaurus II by Grady Ward, 1.0:

101 Moby Thesaurus words for "unseasonable":
   ahead of time, anachronistic, antedated, bad, beforehand,
   behind time, behindhand, dated, deplorable, early, foredated,
   futile, ill-adapted, ill-advised, ill-assorted, ill-chosen,
   ill-considered, ill-fitted, ill-matched, ill-seasoned, ill-sorted,
   ill-suited, ill-timed, impolitic, improper, inadmissible,
   inadvisable, inapplicable, inapposite, inappropriate, inapt,
   inauspicious, incongruous, inconvenient, inept, inexpedient,
   infelicitous, inopportune, intempestive, intrusive, irrelevant,
   late, mal a propos, maladjusted, malapropos, metachronistic,
   misdated, misjoined, mismatched, mismated, misplaced, mistimed,
   off base, out of character, out of date, out of joint,
   out of keeping, out of line, out of phase, out of place,
   out of proportion, out of season, out of time, out of tune,
   overdue, parachronistic, past due, postdated, premature,
   prochronistic, tardy, too late, too soon, unadapted, unapt,
   unbecoming, unbefitting, undesirable, undue, unfavorable, unfit,
   unfitted, unfitting, unfortunate, unhandy, unhappy, unlucky,
   unmeet, unprofitable, unpropitious, unpunctual, unqualified,
   unready, unripe, unseemly, unsuitable, unsuited, untimely,
   untoward, unwise, wrong