The Collaborative International Dictionary of English v.0.48:

Opportune \Op`por*tune"\, a. [F. opporiun, L. opportunus, lit.,
   at or before the port; ob (see Ob-) + a derivative of
   portus port, harbor. See Port harbor.]
   Convenient; ready; hence, seasonable; timely. --Milton.
   [1913 Webster]

         This is most opportune to our need.      --Shak.
   [1913 Webster] -- Op`por*tune"ly, adv. --
   Op`por*tune"ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

opportuneness
    n 1: timely convenience [syn: opportuneness, patness,
         timeliness] [ant: inopportuneness, untimeliness]