Collocation and word choice – select the best noun to fit the phrase "on the ____ of his fiftieth birthday". Sentence: "The family gave father a gold watch on the ____ of his fiftieth birthday." Choose the most idiomatic and precise word.

Given data

• Sentence context: marking a specific milestone date (fiftieth birthday).
• Options: time, event, occasion, celebration.

Concept/Approach
The fixed expression is on the occasion of + event. It is the natural, formal collocation used to mark notable dates.

Option analysis
occasion — correct collocation: "on the occasion of his fiftieth birthday." ✅time — too general; "on the time of" is unidiomatic.event — a count noun but the fixed phrase is not "on the event of".celebration — names the act of celebrating, not the marked date; typical preposition would be "at the celebration".

Usage tip
Set phrase: "on the occasion of + noun phrase".

Final Answer
occasion