Use the BP VBO called Utility - File Management. There is an action called Get Files, where you provide the folder path, the search pattern needed to find the files, and it outputs information regarding each file it finds into a collection. The output data fields include the Path, Folder, Name, Extension, Created DateTime, Last Accessed DateTime, Last Written DateTime, Read Only, and Bytes.
You could use the Created/Accessed/Last Written dates to determine the timeframe, or you could parse the date from within the Name value. I recommend looping through the Files collection and testing each file to see if it is a current month file. If it is, do your process steps. If it isn't, go to the end of the loop and try the next filename.