I have 12 worksheets, one for each month, in the same spreadsheet file. I
have formatted the first one, but how can I copy the formatting (ONLY the
formatting) through to the others without having to do each one separately?

If you click the intersection between the row and column identifiers in
your formatted sheet, this will highlight the complete worksheet. Then
click the Format Painter icon, click the second sheet tab and click in
cell A1. Repeat this ten times.

Pete