Source : Webster's Revised Unabridged Dictionary (1913)

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.

         This is most opportune to our need.      --Shak.
   -- {Op`por*tune"ly}, adv. -- {Op`por*tune"ness}, n.

Source : WordNet®

opportunely
     adv : at an opportune time; "he arrived rather opportunely just
           when we needed a new butler" [ant: {inopportunely}]