The Collaborative International Dictionary of English v.0.48:

Seasonable \Sea"son*a*ble\, a.
   Occurring in good time, in due season, or in proper time for
   the purpose; suitable to the season; opportune; timely; as, a
   seasonable supply of rain.
   [1913 Webster]

         Mercy is seasonable in the time of affliction.
                                                  --Ecclus.
                                                  xxxv. 20.
   [1913 Webster] -- Sea"son*a*ble*ness, n. --
   Sea"son*a*bly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

seasonableness
    n 1: being at the right time [syn: seasonableness,
         timeliness] [ant: unseasonableness, untimeliness]