Let's Encrypt is awesome new free Certificate Authority. It's possible to make it fully automated, but if you fail to configure renewal properly, it will automatically email you before it's too late. Kudos to Let's Encrypt team.
More specifically 20 days before your domain expires Let's Encrypt will send you this email:
Your certificate (or certificates) for the names listed below will expire in 19 days (on 01 Sep 16 05:35 +0000). Please make sure to renew your certificate before then, or visitors to your website will encounter errors.
And the list of domains soon-to-have-expired-certificate follows.