this is what i've come up with years ago:

it returns Monday as 1, Teusday as 2 etc..., regardles of @@Datefirst.

declare @Date datetime

select @Date = '2009-12-17'

selectcasewhen (datepart(weekday,@Date) + @@Datefirst - 1)%7 = 0

then 7

else (datepart(weekday,@Date) + @@Datefirst - 1)%7

end