I think something like this is what you're looking for. I didn't do extensive testing as I wrote it at work. I hope it helps.

CREATE FUNCTION [dbo].[WeekdaysInInterval]

( @StartDate SMALLDATETIME, @EndDate SMALLDATETIME, @day_of_week VARCHAR(9) )

RETURNS INT

AS

BEGIN

DECLARE @found BIT, @num INT

SET @found = 0

--find first occurence of day of week in interval

WHILE @found = 0 AND (@StartDate < @EndDate)

BEGIN

IF DATENAME(weekday,@StartDate) = @day_of_week

BEGIN SET @found = 1 END

ELSE

SET @StartDate = DATEADD(day,1,@StartDate)

END

--find number of weeks with this day of week included

SET @num = (DATEDIFF(day,@StartDate, @EndDate) / 7) + 1

RETURN @num

END