Dynamic Average Formula across multiple worksheets

Hello Everyone

I have searched the forum for the better half of the day, with no resolve. So if there is an answer out there, I apologize for wasting everyone's time.

If you take a look at the workbook "Running Average" on the "Dec 16" Worksheet "Cell C56". I have this formula:
=AVERAGE('Apr 16:Dec 16'!M34), which works great. I know that Cell M34 looks empty, but there is a formula with a hiddne format

What I need is:

as I add a new month "Jan 17" I would like the formula to automatically change from =AVERAGE('Apr 16:Dec 16'!M34) to =AVERAGE('Apr 16:Jan 17'!M34)

I have tried to have a formula on a different worksheet and have Cell C56 reference the formula. But I end up getting the same number across all months, instead of getting the average leading up to that month.

Please let me know if anyone out there know, thanks!