Copying an Aura with Mirage Mirror
Your Mirage Mirror will go to your graveyard.

This is covered in the rulings on Mirage Mirror's Gatherer page:


  If Mirage Mirror becomes a copy of an Aura, it’s put into its owner’s graveyard unless it’s somehow attached to an appropriate object or player already. If it becomes a copy of an Equipment and is attached to a creature, it’ll become unattached when it becomes a non-Equipment artifact again.


Which is a natural consequence of the following rule (emphasis mine):


  704.5m: If an Aura is attached to an illegal object or player, or is not attached to an object or player, that Aura is put into its owner’s graveyard.


In order for the Mirage Mirror to not be moved to the graveyard you would first need to attach it to the land some other way. If you can find a way to attach Mirage Mirror to the land first, and then copy the Market Festival, the Mirage Mirror will stay attached and not go to your graveyard.