Ideally, mussels should be cooked on the same day as purchase.
To keep for an extra day, remove from any sort of packaging (like a plastic bag or butcher's wrap) that will impede oxygen flow (the net bag is fine) and transfer to a bowl. Cover with a damp tea towel or paper towel and refrigerate. As always, be sure to discard any mussels that are not closed or do not close when firmly tapped.