I have a excel workbook with 15 worksheets and every month when I
produce the annualized turnover I have to go into each header on each
sheet to update the month. Do you know of a way using VB or a DATE
FUNCTION I can automate the template to have the current month in the
header? For example: Turnover October 2008

If by "header" you mean the first row of the worksheet, you can choose one
sheet where the current month is entered, and have formulas in all the
others that refer to that one.

On the other hand, if the "header" is what's set up using
File Page setup Header/Footer
then you can select all worksheets at once beforehand, and the update will
apply to all the worksheets. One way to do this is to right-click on one of
the tabs (at the bottom of the Excel window) and choose "Select All
Sheets."

(I have Excel 2003.)