|Click the first tab in the workbook.|
Shift-click the last tab in the workbook.
This will select all tabs and group the worksheets.
Click View...Header and Footer and enter your text.
The Header and Footer will be applied to all sheets.
Make sure to ungroup the sheets by clicking a tab other than the active tab or by right-clicking the tabs and choosing "Ungroup Sheets". If you don't, any changes you make to the active sheet will be made in all sheets.
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