You cannot use a formula to define a dynamic param but you can create a param that you just add the strings of 'Tu-Th' and 'Fr-Sa' then in your select expert:
({?Param}='Tu-Th' and DayOfWeek({FixedSites2009.Date}) in [3,4,5])
or
({?Param}='Fr-Sa' and DayOfWeek({FixedSites2009.Date}) in [6,7])
Edited by DBlank - 30 Mar 2010 at 7:14am