Simple Powershell Question

I have a simple powershell script that performs basic tasks on certain days of the month (1st, 16th, etc). I need to perform tasks on the last day and next to last day of the month but since these can be different month to month I'm not sure how to accomplish that. I've attached a screenshot showing what I have now and would like to know how to add the logic to perform tasks on the last and next to last days of the month. Thanks!