How to make Opportunity Revenue Stream field read only automatically at the end of the month.

Hi all,

I need to make the opportunity revenue stream field which is used to capture month-wise projection read only at the end of the month for that particular month. For example: The opportunity revenue stream field used to capture the revenue projection for the month of May should be made read only on 1st June.

Could someone please help me understand how this can be done using workflow as I do not want to do this manually after every month end?

Thanks in advance for your help.