I would setup a daily subscription, and make sure that you have the "send this report even when there are no results" unchecked. You can then create a formula checkbox field for example on your records with a formula like LastDayOfMonth([DateFieldName]). You could have more to the formula if you wanted to only include records where the status wasn't completed, or where the assigned to field is a specific person, etc. to include the records you want in the report. Last, create a report filtering where that formula checkbox field is checked. Then your subscription will check every day, but only trigger the e-mail on the last day of the month.