oc·ca·sion 
(ə-kāzhən)
(ə-kāzhən)n.
1. An event or happening, or the time of an event or happening: On several occasions, we saw him riding a motorcycle.
2. A significant event, especially a large or important social gathering: The reception proved to be quite the occasion.
3. A favorable or appropriate time or juncture: saw the layoff as an occasion to change careers. See Synonyms at opportunity.
4.
a. A cause of or reason for something: a trade disagreement that furnished the occasion for war. See Synonyms at cause.
b. A need created by a particular circumstance: "He must buy what he has little occasion for" (Laurence Sterne).
5. occasions Archaic Personal requirements or necessities.
tr.v. oc·ca·sioned, oc·ca·sion·ing, oc·ca·sions
Idioms: To provide occasion for; cause: "The broadcast and its immediate aftermath occasioned a cascade of media commentary" (Lewis Sorley).
on occasion
From time to time; now and then.
rise to the occasion
To find the ability to deal with an unexpected challenge.
take the occasion
To make use of the opportunity (to do something).
[Middle English, from Old French, from Latin occāsiō, occāsiōn-, from occāsus, past participle of occidere, to fall : ob-, down; see OB- + cadere, to fall; see kad- in the Appendix of Indo-European roots.]