Since you already have a defining monthly column (Col A), get rid of the
other 11 tabs and use just the one. Once you have everything in one tab, a
pivot table can be defined anyway you like. At worst, you can copy/paste the
original tab with all the details and delete rows you do not need and create
a new pivot table. To me the key concept is to use just the one tabbed
worksheet to accumulate all your information.