"At various occasions" versus "on various occasions"

Which of these sentences is correct?

The importance to eat fruits and vegetables of different colors has been demonstrated at various occasions.
The importance to eat fruits and vegetables of different colors has been demonstrated on various occasions.

The preposition that goes with "occasion" is usually "on", and never "at" (unless by "occasion" one means an event or gathering, which is not the case here), so "On various occasions" sounds more natural. Note, similarly, that we can also say:

I like to eat broccoli on occasion.

but never:

I like to eat broccoli at occasion.

I would also suggest "many times" or "several times" as alternatives to "on various occasions".

"Demonstrate at various occasions" would most naturally be understood to refer to demonstrations at parties marking important occasions - weddings and the like.

"Demonstrate on various occasions" suggests that on definite occasions such a demonstration had been given.