Mussels are best when cooked on the same day of purchase. But if you store them, they can stay fresh for about two days in the fridge. In the fridge, they should be stored in a bowl covered with a damp towel or damp paper towel. Drain any water that is collected. It is essential to check the mussels before cooking by giving them a smell test and discarding any with cracked shells. If a shell is slightly open, tap against a hard surface like a bowl or counter. If it closes, it is still alive and good to use; if it stays open, discard it.