Originally Posted by Pecoflyer

If you use 3D references to sum the same range in multiple consecutive sheets you an use following trick
create a sheet called "Start", leave it blank and place it before all the sheets containing data
create a sheet called "end", leave blank, place after all sheets containing data
Now build your formula with the shift key from "start" to "end"
Whenever a new month is added place the new sheet between "start" and "end".
That's it