Straight forward way;
Drag order date onto the color button of the marks card, then right click the blue pill which is added, select More, Weekday. Then color your weekdays different to the weekend.
A better solve is to create a calculated field to determine if it's a weekday or not as:
if datepart('weekday',[Order Date]) = 1 or datepart('weekday',[Order Date]) = 7 then 'Weekend' else 'Weekday' end
Drop that on the color shelf.