The Collaborative International Dictionary of English v.0.48:

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; event.
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            The unlooked-for incidents of family history, and
            its hidden excitements, and its arduous occasions.
                                                  --I. Taylor.
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   2. A favorable opportunity; a convenient or timely chance;
      convenience.
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            Sin, taking occasion by the commandment, deceived
            me.                                   --Rom. vii.
                                                  11.
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            I'll take the occasion which he gives to bring
            Him to his death.                     --Waller.
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   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.
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            Her beauty was the occasion of the war. --Dryden.
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   4. Need; exigency; requirement; necessity; as, I have no
      occasion for firearms.
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            After we have served ourselves and our own
            occasions.                            --Jer. Taylor.
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            When my occasions took me into France. --Burke.
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   5. A reason or excuse; a motive; a persuasion.
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            Whose manner was, all passengers to stay,
            And entertain with her occasions sly. --Spenser.
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   On occasion,
      (a) in case of need; in necessity; as convenience
          requires. "That we might have intelligence from him on
          occasion," --De Foe.
      (b) occasionally; from time to time; now and then.
          [1913 Webster +PJC]

   Syn: Need; incident; use. See Opportunity.
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WordNet (r) 3.0 (2006):

on occasion
    adv 1: 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, on occasion, once in
           a while, now and then, now and again, at times,
           from time to time]