Let's encrypt SSL certificates will get expired after 90 Days of installation and you must renew it before it get expired.
Certbot Renew Command
$ sudo certbot renew --dry-run
Certbot Auto Renew Cron Job
$ cat /etc/cron.d/certbot
Content:
# /etc/cron.d/certbot: crontab entries for the certbot package
#
# Upstream recommends attempting renewal twice a day
#
# Eventually, this will be an opportunity to validate certificates
# haven't been revoked, etc. Renewal will only occur if expiration
# is within 30 days.
SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin
0 */12 * * * root test -x /usr/bin/certbot -a ! -d /run/systemd/system && perl -e 'sleep int(rand(43200))' && certbot -q renew
How Certbot's Auto Renew Script Works ?