Hi maybe someone can help me.

I want to calculate the day previous to a given date ...
for example if I enter
06/11/2007 I want to return 05/11/2007
However if the day entered is a Monday I want to return the previous
Friday's date
for example if I enter
05/11/2007 I want to return 02/11/2007

I have been trying an IF but to no avail. Any suggestions greatfully
received :) I figure it must be possible. Thanks