“Except of course, when the said apparition is Mary.”
No Catholic believes that one can summon the Blessed Virgin. That would be necromancy.
Mary appears when and where and to whom it pleases God that she should, and perhaps sometime she whispers a word in His ear about some poor soul whose prayers have moved her merciful heart. I don’t know what the criteria are. I don’t know why God does such things.
I do know, however, that He does do such things, and more frequently than I once thought.