on occasion
  偶爾,有時,間或,必要時
  Oc·ca·sion n.
  1. A falling out, happening, or coming to pass; hence, that which falls out or happens; occurrence; incident; event.
     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, (a) in case of need; in necessity; as convenience requires. “That we might have intelligence from him on occasion,”   -- (b) occasionally; from time to time; now and then.
  Syn: -- Need; incident; use. See Opportunity.
  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]