How to calculate weekly average from daily data

Question

Tableau 8.1.
Sample Data: 

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.

What I want?

I would like to get the weekly average of each month
Somewhat 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 you 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

You can create these calculated fields:

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).

or 

simply create one calculated field.

WINDOW_AVG(SUM([Sales]))

and drag your Date pill onto the rows shelf twice, one aggregated to day, the other to week. Make sure pills are set to discrete (blue) and drag your Sales measure onto the shelf, also as discrete.