You can keep fresh mussels in the fridge for approximately 2 to 3 days. After that they will start to turn bad. In order to keep fresh mussels in the fridge for 2 to 3 days you need to store them using the right method.
Once you're ready to eat the mussels, you can thaw them in the refrigerator overnight. Another option is to place them in a bowl of cold water for about 1 hour. After thawing, you can safely keep the mussels in your refrigerator for up to 2 days before cooking and eating them.
The mussels will produce a small amount of liquid each day. If you don't keep them in a self-draining container (such as a colander), this liquid will accumulate in the container. Pour out any liquid each day to keep your mussels fresh and healthy for as long as possible. [8]
Myth: Mussels have gone bad if they are open before cooking. Fact: Mussels that are open before cooking are most likely still alive. Give them a tap either with your finger or on the side of a bowl and wait for the shell to close up. If the shell does not close after tapping, then discard. Should fresh mussels be open or closed?