Formula To Automatically Change External Reference Based on Date

I have a YTD report that has columns for each month. The KPIs listed for each month are each pulled from individual month-end reports which I drop into the folder at the end of the month. I want to automate the external references to pull the data from the correct month based on the column header. For example, at the end of May, I will drop the May report in the folder. I want my document to pull the correct data based on the column header (May, in this case) so it automatically knows to change the external reference to "May 2013 KPI Report" instead of "April 2013 KPI Report."

Report names and cell references are constant through to each report -- what's in A4 in one report will be in A4 in all reports.

When I "front-load" the formulas to anticipate files that are not there yet, it causes #REF errors that carry through the document and mess up totals. Is there any way to possibly automate this process?

So I've given up on the previous. Now I just need to create a macro that will c&p data from previous worksheets over data in report. For example, for each month, I have worksheets named:
May KPIs1
May KPIs2
May KPIs3

The May KPI reports include forecasts for May to year-end. The copy and paste is simple enough. I'm just trying to figure how to get the macro to "start" in the right cell without a direct cell reference? See below:
The report looks like this:
May June July August
100 200 300 400
100 200 300 400
100 200 300 400

I take the actual for May from the May report and take the forecast for June - August also from the May report. But once the June report is added, how do I make the macro start at June automatically (preferably without activecell ref). Is there anyone that can help me with this?

Very much appreciated!!!