This code is indipendent from datefirst:

Monday is 0 and Sunday is 6

SELECT cast(cast('20100215' as datetime) as int)%7

SELECT cast(cast('20100216' as datetime) as int)%7

SELECT cast(cast('20100217' as datetime) as int)%7

SELECT cast(cast('20100218' as datetime) as int)%7

SELECT cast(cast('20100219' as datetime) as int)%7

SELECT cast(cast('20100220' as datetime) as int)%7

SELECT cast(cast('20100221' as datetime) as int)%7