Sum values until the active sheet

Hello all,
This is my first post here. I've searched the entire forum but couldn't find an answer for my situation. This is what I am trying to do;

My sheet names are the dates of the month.. 07 Feb,08 Feb,09 Feb etc.. I want to sum up the values in a cell(for example L41) in every sheet from the beginning until the sheet that I am in (activesheet). The reason is I am trying to find the average value until that day of the month.
What I did is;

=SUM('07 Feb:Sheetname()'!L41)/(SheetNum()-1)

But it doesnt let me use the Sheetname macro in the sum function..
So what can I do to automize the function so that it will add all the values from the start(07 Feb) until the active sheet that I am in?

Help much appreciated !

Thank You