Column A Column B
Date Week Number
Mon, Jan 08, 2007 2 =WEEKNUM(A2,1)
Mon, Jan 08, 2007 2 =WEEKNUM(A3,1)
Mon, Jan 08, 2007 2 =WEEKNUM(A4,1)
Tue, Jan 09, 2007 2 =WEEKNUM(A5,1)
Tue, Jan 09, 2007 2 and so on
Tue, Jan 09, 2007 2
Wed, Jan 10, 2007 2
Wed, Jan 10, 2007 2
Wed, Jan 10, 2007 2
Wed, Jan 10, 2007 2
Wed, Jan 10, 2007 2
Thu, Jan 11, 2007 2
Thu, Jan 11, 2007 2
Thu, Jan 11, 2007 2
Thu, Jan 11, 2007 2
Fri, Jan 12, 2007 2
Fri, Jan 12, 2007 2
Sun, Jan 14, 2007 3
Sun, Jan 14, 2007 3
Sun, Jan 14, 2007 3
Mon, Jan 15, 2007 3
Mon, Jan 15, 2007 3
Mon, Jan 15, 2007 3
Mon, Jan 15, 2007 3
and so on

So now I've to count working days in the weeks 1, 2, 3, ..........53
Same date entry as 1 day.
Something similar to count by month:
=SUMPRODUCT(--(YEAR(trips!$A$2:$A$1200)=2007),(MONTH(trips!$A$2: $A$1200)=1)
/COUNTIF(trips!$A$2:$A$1200,trips!$A$2:$A$1200&""))
Thanks in advance. Igor.