Aerith Gainsborough is a simple flower vendor who happens to be the last living Cetra. She resolves to save the world from Sephiroth, only to die by his hands. As Sephiroth casts meteor, Aeris’s spirit commands the Lifestream to push Meteor back. Holy then destroys the Planet. Aerith stays in the Lifestream, trying to find a way to save the world from Geostigma.
When does Cosmos call Aerith to the Lifestream? Cosmos did not call her, but she arrives in World B after her death. From a gameplay standpoint, she’s an Assist-only character and we can’t play her character. However, she hasn’t completely merged with the Lifestream. She has enough individuality to appear in form to the characters she assists. Thus, the time most likely takes place during the events in Advent Children: Final Fantasy VII.
While Cloud and the others can sense her presence, Sephiroth is able to remember her. He uses “Hell’s Gate” repeatedly on Aerith when he sees her. Sephiroth uses the Hell’s Gate move to kill Aerith at the Forgotten Capital.