Still incorrect formula.
Temporarily I got formula
Week 1:
=SUMPRODUCT(--($A$2:$A$100=DATE(2007,1,1)),--( $A$2:$A$100<=DATE(2007,1,6))
/COUNTIF($A$2:$A$100,$A$2:$A$100&""))

Week 2:
=SUMPRODUCT(--($A$2:$A$100=DATE(2007,1,7)),--( $A$2:$A$100<=DATE(2007,1,13))
/COUNTIF($A$2:$A$100,$A$2:$A$100&""))

and so on.

If you will have some idea, please let me know.
Thanks again for your time.
Sincerely, Igor.

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