Source : Webster's Revised Unabridged Dictionary (1913)

Occasion \Oc*ca"sion\ ([o^]k*k[=a]"zh[u^]n), n. [F. occasion, L.
   occasio, fr. occidere, occasum, to fall down; ob (see {Ob-})
   + cadere to fall. See {Chance}, and cf. {Occident}.]
   1. A falling out, happening, or coming to pass; hence, that
      which falls out or happens; occurrence; incident.

            The unlooked-for incidents of family history, and
            its hidden excitements, and its arduous occasions.
                                                  --I. Taylor.

   2. A favorable opportunity; a convenient or timely chance;
      convenience.

            Sin, taking occasion by the commandment, deceived
            me.                                   --Rom. vii.
                                                  11.

            I'll take the occasion which he gives to bring Him
            to his death.                         --Waller.

   3. An occurrence or condition of affairs which brings with it
      some unlooked-for event; that which incidentally brings to
      pass an event, without being its efficient cause or
      sufficient reason; accidental or incidental cause.

            Her beauty was the occasion of the war. --Dryden.

   4. Need; exigency; requirement; necessity; as, I have no
      occasion for firearms.

            After we have served ourselves and our own
            occasions.                            --Jer. Taylor.

            When my occasions took me into France. --Burke.

   5. A reason or excuse; a motive; a persuasion.

            Whose manner was, all passengers to stay, And
            entertain with her occasions sly.     --Spenser.

   {On occasion}, in case of need; in necessity; as convenience
      requires; occasionally. ``That we might have intelligence
      from him on occasion,'' --De Foe.

   Syn: Need; incident; use. See {Opportunity}.

Source : WordNet®

on occasion
     adv : now and then or here and there; "he was arrogant and
           occasionally callous"; "open areas are only
           occasionally interrupted by clumps of trees"; "they
           visit New York on occasion"; "now and again she would
           take her favorite book from the shelf and read to us";
           "as we drove along, the beautiful scenery now and then
           attracted his attention" [syn: {occasionally}, {once in
           a while}, {now and then}, {now and again}, {at times},
           {from time to time}]