Continuing the discussion from Conceptual Issues with operational handling of letsencrypt:
Since you have to renew your certificates at least every 90 days and there’s currently no support for renewal in the official client, many users have resolved this issue by creating some wrapper script and putting that into cron. This may lead to problems, as cron may not be executed or something else.
It always should be best, if you monitor the state of your certificates in a periodic way too see if there’s a problem. This can be very easy if you have just one server (just connecting to the page once a day or week). But if you have multiple sites, you may want to have something more sophisticated. Maybe some standalone software or a plugin for one of the commonly used monitoring softwares (nagios, zabbix etc.)
I propose here a list of such monitoring software. Post your software below so that I can update this list accordingly.
GlobalSign Inventory: https://www.globalsign.com/en/blog/new-certificate-inventory-tool/
Certificate Expiry Monitor: https://certificatemonitor.org/