If you are using Analytics > Performance reports your best option is to use the "Last work week" filter.
If I were you I would use Analytics > Business Analytics. This is the newer version of Performance reports which is more customizable. Here you are able to select a monthly date range, and deselect the weekends. As you can see in pic below I have selected Last month, but at the very bottom I have deselected Sunday and Saturday.
Pro tip: don't use the global filters which Business Analytics offers. Instead configure filters for each widget individually.
If that suggestion doesn't meet your needs, please check out the App Market Brightmetrics for RingCentral solution. We can tailor reports in a very granular fashion and provide additional features as well. Let me know if you have any questions. Tim Ganun tganun@brightmetrics.com.