occasion-n; 4 Senses

Sense Number 1: a specific time period, happening

Commentary: OCCASION[+time][+period][+specific]
NOTE: refers to some point in time

On at least one occasion we worked all night to finish a project on time.
The company gave him a gold watch on the occasion of his retirement. (a specific date, perhaps also Sense 2, if a party is implied)
During the crisis John rose to the occasion and took a leadership role.
On occasion the state was asked to intervene.

WordNet 3.0 Sense Numbers: 1, 4

Sense Number 2: a social affair or function

Commentary: OCCASION[+event][+social][+function]
NOTE: usually implies multiple participants at a social event

The drugstore sells cards for all occasions.
Mary wore her black cocktail dress on special occasions.
These formal occasions tend to go on too long.
His funeral will be a somber occasion.

WordNet 3.0 Sense Numbers: 2

Sense Number 3: an opportunity to do something

Commentary: OCCASION[+state][+situation][+opportunity][+action]
NOTE: occasion is -not- a cause, but a chance to do something
NOTE: no agent/actor supplies a cause

There was never an occasion to talk to him in person.
Elections are an occasion for registering one's protest vote.
You may not get another occasion to see such a spectacular meteor shower.

WordNet 3.0 Sense Numbers: 5

Sense Number 4: the cause or reason of something, inciting event

Commentary: OCCASION[+relation][+role][+cause] [+state][+inciting]
NOTE: 'occasion' serves as a cause leading to a consequence
NOTE: Agent/Actor give/have OCCASION{+cause} (INFINITIVE NP){+consequence}

I've never had occasion to complain about their service until now.
The insulting remark gave the ambassador occasion to slap the man's cheek.
The servant never provided them with an occasion to distrust him.

WordNet 3.0 Sense Numbers: 3