Where

Google ngrams is an excellent tool for this type of question. It shows that the collocation of "occasion when" is more common in print than "occasion where." Additionally, it shows that "situation where" is more common that "situation when."

When is correct. "On various occasions" refers to different times or situations, when these situations happen. Occasion is a point in time or specific event, in simple terms, it's a time word. Where could be used to refer to non-physical places. "We need to avoid a situation where no one is in charge." But given "Occasion" has a temporal identity, it would be grammatically and more conceptually precise.

I was thinking that when we do word-by-word translations, the words refer to adverbs. When is referring to an adverb of time, where refers to an adverb of manner.