Salesforce has sent many of our customers the following e-mail:
Subject: SFDC Expiring Certificate Notification
You have one or more certificates in your Salesforce org [NAME + Orgid] that will expire soon. Please review the list below and visit Certificate and Key Management from Setup to make an update.
You do not have to take any action regarding to this notification.
The certificate in the mail refers to a certificate for SSO. If no use is made in any case this notification is not applicable to you. And, after conducting several tests, however, we have concluded that this does not affect any customer who do use SSO.