Nagios OK notification at the beginning of the availability period
Solution 1:
Looks to me like a perfect opportunity to have cron schedule a custom service notification, which is basically a gratuitous notification describing the current state of affairs. You can do this through the external command interface, with eg
echo "[1495974284] SEND_CUSTOM_SVC_NOTIFICATION;hostname;service name;0;username;Comment eg morning notification" > /var/nagios/rw/nagios.cmd
Documentation on this external command can be found here.