Averaging data from months prior to this month (Excel 2013)

I'm working on financial spreadsheet for the year of 2013. My column headers are the months, the row headers are budget categories, and the data is the amount I've spent per month in each category.

What I would like to do is create a function for each category that averages out the months prior to this month. So, since we are in march now, it would average the values entered under Jan, Feb, Mar, Apr. Then when it gets to June, it would average the values from Jan, Feb, Mar, Apr, May.

Thoughts?