sliding aggregates

I have the following sql statement to calculate sliding aggregates. But days between AI3_DATE-4 and AI3_DATE might not exactly equals 5(only business days).
How can I specified where clause to meet the need?

select a.AI3_DATE, avg(b.AI3_INDEX)
  from AI3 a, AI3 b
 where (b.AI3_DATE between a.AI3_DATE-4 and a.AI3_DATE)  group by a.AI3_DATE Received on Fri Oct 21 2005 - 00:42:29 CDT