The emailer system has a special feature such that if you enter 31st as the email date then the system will send out at the end of every month, regardless of the number of days in that month.
To send an email at End of Month, simply select the 31st. There is a function built in to the emailer to check if the current date is the last day of the month, if so, any reports set to 31st will be sent e.g. 28th Feb, 30th April etc.