On his retirement, he handed the business over to his son

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1

On his retirement, he handed the business over to his son.

Question: What does the preposition “on” in front of his retirement in the sentence mean?
Is it okay to use “in” instead?

Thank you!!!

2

Not exactly. ‘On’, or ‘upon’ (which I think is better in this case), means at the moment when he retired. ‘In’ means at some point during his retirement.

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3

“on” , in this case means, “at the beginning of”
“on” , in this case means, “at the commencement of”

It is not okay to use “in” instead of “on” in this case.