Hi Mark,
I assume that you have the month number anywhere on your sheet. Example: Anywhere in "tab3" there should be a cell containing the number "3". Assume the information is provided in cell A1 for each tab.

With the above assumptions, try this formula to refer to cell J10 on the "previous tab":

=IF(A1=1,0,INDIRECT("tab"&A1-1&"!J10"))

Kind regards
Bjorn