Another option is to figure out the first and last day of week within the interval and get the number of weeks . Using iTVF

create function NumDOWBetween (@StartDate DATETIME, @EndDate DATETIME, @thisday varchar(3))

returns table as

return select

nofd = case

when firstthisday > lastthisday then 0

else datediff(week, firstthisday, lastthisday) + 1 end

from (

select

wdn = datepart(dw,dateadd(d,n,'20010101')),

wds = datename(weekday,dateadd(d,n,'20010101'))

from (values (1),(2),(3),(4),(5),(6),(7)) as t(n)

) t2

cross apply(

select

firstthisday = case when datepart(dw,@startdate) <= wdn

then @startdate + wdn - datepart(dw,@startdate)

else @startdate + 7 - wdn + datepart(dw,@enddate)

end,

lastthisday = case when datepart(dw,@enddate) >= wdn

then @enddate + wdn - datepart(dw,@enddate)

else @enddate - 7 + wdn - datepart(dw,@enddate)

end

) interval

where left(wds,3) = @thisday;

go