Is this in a function, or in a query?
IN a query, this should work.....
=IIf(Weekday([yourdate],0) Between 1 And 5,"X",IIf(Weekday([yourdate],0)<=7,"Y",Null))
OR
=IIf(isnull(yourdate),null,iif(Weekday([yourdate],0) Between 1 And 5,"X","Y"))
should work similarly in a function?