Hi
You haven't specified which version of Ontap and Protection Manager you are using. so i can't give exact directions
i saw that in On command For 7-mode there is an option in the schedulers for "31/last".
but if you using Cdot system,or have schedules directly in Ontap 7-m it will be using Cron (https://en.wikipedia.org/wiki/Cron) to schedule jobs. and cron do not allow to specify this,
however it does allow to specify the first day of a month. i hope that's good enough.
Gidi