Tableau - weekly average from daily data

Question

Using Tableau 8.1.
Daily data like this:

Date     Sales
1-1-14     $5
1-2-14     $2
...
6-15-14    $15

Which can be aggregated to weeks of course

Week 1   $15
Week 2   $12
Week 3   $10
Week 4   $13

etc.

I would like to get the weekly average of each month
So like this:

          Weekly Average
Month 1     (weekly average of the weekly total sales)  ($15+$12+$10+13)/4 = 12.5
Month 2 
Month 3 

Does that make sense? If ou simply put "average" in the tableau table, it gives the average daily value PER WEEK (or month) < - I don't want that. I want the average of the weekly totals per month.

Answer 1

Quite simple, actually. Create these calculations

Unique Weeks = COUNTD(DATEPART('week',[Date]))
Weekly Average = sum([Sales])/[Unique Weeks]

Then simply report Weekly Average x Month (or Quarter or Year, etc.)
(Note: Some weeks have < 7 days).