15 3 * * * /usr/bin/certbot renew --quiet --deploy-hook 'systemctl reload nginx'

A crontab line: at 03:15 daily, run certbot renew quietly and reload nginx only when a renewal actually happens (the --quiet and --deploy-hook pattern from the Certbot docs). The five fields are minute, hour, day-of-month, month, day-of-week. Note that the packaged certbot.timer already does this on Ubuntu — add this only if you manage cron yourself.