previous occasion = earlier/ past special time/ event/ ceremony/celebration

It doesn't necessarily have to be an event. It can be used for any time. It can also be used to mean "at a previous instance of this thing happening" e.g. The last goal was an amazing piece of work by Ronaldo. It was visibly much better than the previous goal. On the previous occasion, it was more about better passing and team play, but on this occasion, it was completely the work of the player himself.

its an occasion that has been done already.

It means on a previous/past meeting or an event!