Need magic for a moving statistic

Hi,

I need some magic for a moving statistic that works on a rather big 
table starting at a given date within the table up until now.
The statistic will count events allways on fridays over periods of 2 
weeks before ... biweekly?
So I'd like to get a line every 2 weeks for everthing between.

I sadly don't know how to spell that does:

collect data   where  insert_date  between  friday1   and   friday1 + 
inteval '2 week'
collect data   where  insert_date  between  friday1 + inteval '2 week'  
+ inteval '1 second'   and   friday1 + inteval '4 week'
collect data   where  insert_date  between  friday1 + inteval '4 week'  
+ inteval '1 second'   and   friday1 + inteval '6 week'

Is this possible ?


regards