Here is the define Nagios command you need to use to monitor the expiration of your SSL certificates:
command_line /usr/lib/nagios/plugins/check_http --ssl -C 7 -H '$HOSTADDRESS$' -I '$HOSTADDRESS$'
As you can see it is just the default check_http Nagios-plugin with a few arguments.
--ssl tells the plugin that we use SSL
-C is the certificate or check certificate argument followed by the number of days we will get warned before the certificate expires
-H is the hostname/address argument (could also be aarvik.dk)
-I is the optional IP-address argument
$HOSTADDRESS$ is a "Standard Macro" in Nagios: http://nagios.sourceforge.net/docs/3_0/macrolist.html
Read more about the check_http plugin here: