occasion (n.) Look up occasion at Dictionary.com
late 14c., from O.Fr. occasion, from L. occasionem (nom. occasio) "opportunity, appropriate time," from occasum, pp. of occidere "fall down, go down," from ob "down, away" + cadere "to fall" (see case (1)). The notion is of a "falling together," or juncture, of circumstances. The verb is first attested 1520s.