I'm not sure exactly how you want the view to work out. The basic view will use a WINDOW_MAX(SUM([Sales])) with the Compute Using set to the Month of Order Date, I set up a couple of variations on this - one showing all the months, the other that just returns a single row so you can have it in a view without overlapping text.
However, interactions between table calculation Compute Using (addressing and partitioning) and drill-downs are not able to be specified. So if you just drilldown to week with the Compute Using set to Order Date, the Compute Using changes to the week as well and the WINDOW_MAX/MIN results will change. There's a set of workarounds using parameters in place of the drill-downs, but that means that you'd no longer be using the built-in drill-down functionality.
Thank you. The part I was missing was adding the Month to the level of detail. This is exactly what I needed! (only wish I had asked sooner)