DATEFIRST keyword use to reset first day of week in SQL Server and @@DATEFIRST return the current value for the session of DATEFIRST.
As we know SQL Server set default language US English and SQL Server sets by default DATEFIRST to 7 (Sunday).
SET DATEFIRST 6
If we execute above query it will set Saturday as the first day of week.
@@DATEFIRST will returns the current value of SET DATEFIRST for the session.
SET LANGUAGE french
--result will return 1 (Monday)
SET LANGUAGE us_english
--result will return 7 (Sunday)