Dimension member Properties Value count

We have a "Time" dimension with "Year->Month->Week" and Date ID as Key column. FYI... Date ID is not exposed as attribute or in hierarchy as the lowest level in the hierarch is Week. However, "Date ID" has a property called"Working Day", which has value 0 or 1. What we need to do is to count the number of working days for a selected period. Can someone suggest MDX to consider. Example one of the metric could be Sales per week, Sales per month. FYI... Use could pick any combination i.e. if the user picks 2 months, he sould get sales divided by "Working days" in those 2 months.

Thanks

Krish