Well it sounds like you have already accomplished the first step in the process--defining exactly what you need to do.
Having the date information in the file names should make it easy to locate the files you need. When you say these are spreadsheet files are they Excel formatted files, or tab-delimited files that Excel can open, or (heaven forbid) a mixture of the two?
Structurally, I would concentrate first of writing a VI that accepts a filename and path as an input and does all the required processing on that file. Once you have that function written, simply put it in a loop and do it 30 times with the appropriate filenames.
Are there specific things you are wanting to know how to do?
Mike...