Olaf,

I got it to work, sort of. (Well, in my bar graph.)

I created a simple bar graph with (Date, Value), so the x-axis represents dates. If you right-click on a bar in the graph, you can change the color rule. Go to Series Properties, and Fill. Then enter an expression like this one:

=IIF(Weekday(Fields!TheDate.Value)>=6,"Red","Automatic")

{6=Saturday, 7=Sunday}

Hope this helps,

Pieter