I am assuming you have a new worksheet for each month. On the latest Months
worksheet on the beginning balance line you should put a formula to reference
the ending balance from the last months.

For example if the last months worksheet was Sheet1 and the ending balance
was on cell B20 then on the new months worksheet in the cell that you want
the ending balance from last month type the following formula

=Sheet1!B20

If you're willing to use a User Defined Function.......

Function PrevSheet(rg As Range)
   n = Application.Caller.Parent.Index
   If n = 1 Then
       PrevSheet = CVErr(xlErrRef)
   ElseIf TypeName(Sheets(n - 1)) = "Chart" Then
       PrevSheet = CVErr(xlErrNA)
   Else
       PrevSheet = Sheets(n - 1).Range(rg.Address).Value
   End If
End Function

Say you have 20 sheets,  sheet1 through sheet20...........sheet names don't
matter.

Select second sheet and SHIFT + Click last sheet

In active sheet B1 enter =PrevSheet(A1)

Ungroup the sheets.

Each B1 will have the contents of the previous sheet's A1

Copy/paste the UDF above into a General Module in your workbook.

If not familiar with macros and VBA, visit David McRitchie's website on
"getting started".

http://www.mvps.org/dmcritchie/excel/getstarted.htm

In the meantime..........

To create a General Module, hit ALT + F11 to open the Visual Basic Editor.

Hit CRTL + R to open Project Explorer.

Find your workbook/project and select it.

Right-click and Insert>Module.  Paste the above code in there.  Save the
workbook and hit ALT + Q to return to your workbook.

Gord Dibben Excel MVP