Select SQL results grouped by weeks

I want to select data from following table group by weeks

 Date       Product Name   Sale
+----------+--------------+-----+
 14-05-11     a             2
 14-05-11     b             4 
 17-05-11     c             3
 19-05-11     a             6
 24-05-11     a             6
 29-05-11     a             6    

Let suppose today is 30-05-11

So my result should look like this.

 Product Name         First Week   Second Week  Third Week
+--------------------+------------+------------+-------------+
   a                      12            6           2
   b                       0            0           4 
   c                       0            3           0  

Will some body guide me to how to write SQL query to achieve this behavior!