occasion

1. 1. An event or happening; an incident.
   2. The time at which an event occurs.
2. A significant event.
3. A favorable or appropriate time or juncture; an opportunity. See synonyms at opportunity.
4. Something that brings on or precipitates an action, condition, or event, especially the immediate cause. See synonyms at cause.
5. Something that provides a reason or justification; a ground.
6. A need created by a particular circumstance: “He must buy what he has little occasion for” (Laurence Sterne).
7. A large or important social gathering.
8. occasions Archaic. Personal requirements or necessities.

To provide occasion for; cause: “The year's annual reports occasion an especially revealing glimpse of how corporations lend . . . embellishment to the stark numbers of the comptroller's office” (Mark Muro).

on occasion

1. From time to time; now and then.

[Middle English, from Old French, from Latin occāsiō, occāsiōn-, from occāsus, past participle of occidere, to fall : ob-, down; see ob– + cadere, to fall.]