I thought of this solution also but what if you're going to be out of the office the 2nd to last day of the month and clearing your calendar for that day? You might want to delegate tasks due that day to coworkers. With this proposed solution, you might overlook the task that is due on the 2nd to last day. In my case, ordering the company lunch for the last day of the month. That would be a bad thing to forget to delegate :(

Nor does it block out time for you to actually do the work on that day before.

This solution doesn't work for workdays if the last day working day of the month is Monday. For example 31/07/2017 is Monday. The reminder will reminder you only a day before i.e. Sunday which is not good. You need to be reminder the workday before which is the Friday 28/07/2017