How do I group date results by week starting on a specific day?

[comfixit]

I have the following query that produces the desired result, except I would like to be able to control which 7 days constitute a grouping. Specifically I would like to run it Friday - Thursday or Monday - Sunday depending on the situation.

Here is the query I have so far:

SELECT COUNT( * ) AS reports_in_week, DATE( DATE_ADD( created, INTERVAL( 1 - DAYOFWEEK( created ) )
DAY ) ) AS Start_Date, DATE( DATE_ADD( created, INTERVAL( 7 - DAYOFWEEK( created ) )
DAY ) ) AS End_Date
FROM posts
GROUP BY WEEK( created )
LIMIT 0 , 30

and here is what some of the results look like:

# 	Start_Date 	   End_Date
3 	2008-12-28 	2009-01-03
22 	2009-01-04 	2009-01-10
13 	2009-01-11 	2009-01-17
10 	2009-01-18 	2009-01-24
11 	2009-01-25 	2009-01-31
3 	2009-02-01 	2009-02-07
1 	2009-02-08 	2009-02-14
2 	2009-02-15 	2009-02-21
12 	2009-02-22 	2009-02-28

So what I need to know is how to manipulate the query so I can control the start date to be Monday or Friday (or whatever depending on what is needed)

[crmalibu]

I think you could group by this

DATE_FORMAT(CASE
            WHEN DAYOFWEEK(created) < 4 
            THEN created - INTERVAL 1 WEEK
            ELSE created
            END), '&#37;Y%U')