"Once" does not necessarily refer to any occasion. "Once" is closely tied to the number "one".
For example, you can say "the letter 'e' appears once in the word 'eat'" but it would sound funny (but not be wrong) to say "the letter 'e' appears on one occasion in the word 'eat' ".
As another example, you can say "you can only go swimming once this week". It would sound odd (but not be wrong) to say "you can only go swimming on one occasion this week". It would sound odd because the word "occasion" has no clear reason to be in the sentence.