occasion

occasion
oc·ca·sion¹  /əˈkeɪʒən/ n

on ... occasion  [C]

a time when something happens

1. On one occasion, she called me in the middle of the night.
2. I've seen Jana with them on several occasions.

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occasion + (for sth / doing sth) [singular]

a suitable or favourable time

1. This was the occasion for expressions of friendship by the two presidents.
2. I'll speak to him about it if the occasion arises (= if I get a chance). 

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[C]
a special event, ceremony or celebration

1. They marked the occasion (= celebrated it) with an open-air concert.

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[U] formal
a cause or reason

1. I had occasion to call on him last year.

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if (the) occasion arises
formal if a particular action ever becomes necessary

1. I'm willing to go to court over this if the occasion arises (= if it becomes necessary).

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on occasion (=sometimes =once in a while)

sometimes but not often

1. On occasion prisoners were allowed visits from their families.
2. Herry doesn't like to socialize, but he does go out on occasion.
   

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occasion² v [T] formal

to cause something

1. The flight delay was occasioned by the need for a further security check.

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occasion sb sth

1. Your behaviour has occasioned us a great deal of anxiety.

HINT sense 1
Do not use occasion when you mean 'a time when it is possible for you to do what you want to do'. Use opportunity or chance: Do not waste this opportunity (NOT this occasion).