On the other hand, if you really are presenting a spreadsheet, rather than working with it then here is how to put up a background picture in your spreadsheet.
Most image editors can take a standard image and turn it into a watermark version.
If you have already added the UK holiday dates, then you will need to change the dates already entered in your Outlook Calendar. Double-click the holiday and in Start Time click to select August 31, 1998. Note, it is normal to start your address block with double dash on the first line at the left hand side. Many mail readers understand and separate out the signature. Incidentally, this method uses the same address block for all mail.