Here is an exerpt from Chip Pearson's Site...
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Number Of Mondays In Period
If you need to return the number of Monday's (or any other day) that occur
within an interval between two dates, use the following Array Formula:
=SUM(IF(WEEKDAY(A2-1+ROW(INDIRECT("1:"&TRUNC(B2-A2)+1)))=C2,1,0))
This formula assumes the following:
A2 contains the beginning date of the interval
B2 contains the ending date of the interval
C2 contains the day-of-week number (1=Sunday, 2=Monday,...,7=Saturday)
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Being an Array formula it must be commited with Ctrl+Shift+Enter when it is
typed in...
Here is a link to Chip's site.
http://www.cpearson.com/excel/DateTimeWS.htm