SET DATEFIRST (Transact-SQL)

number | @number_var

Is an integer that indicates the first day of the week. It can be one of the following values.

Value	First day of the week is
1	Monday
2	Tuesday
3	Wednesday
4	Thursday
5	Friday
6	Saturday
7 (default, U.S. English)	Sunday

To see the current setting of SET DATEFIRST, use the @@DATEFIRST function.

The setting of SET DATEFIRST is set at execute or run time and not at parse time.

Requires membership in the public role.

The following example displays the day of the week for a date value and shows the effects of changing the DATEFIRST setting.

-- SET DATEFIRST to U.S. English default value of 7.
SET DATEFIRST 7;

SELECT CAST('1999-1-1' AS datetime2) AS SelectDate
    ,DATEPART(dw, '1999-1-1') AS DayOfWeek;
-- January 1, 1999 is a Friday. Because the U.S. English default 
-- specifies Sunday as the first day of the week, DATEPART of 1999-1-1
-- (Friday) yields a value of 6, because Friday is the sixth day of the 
-- week when you start with Sunday as day 1.

SET DATEFIRST 3;
-- Because Wednesday is now considered the first day of the week,
-- DATEPART now shows that 1999-1-1 (a Friday) is the third day of the 
-- week. The following DATEPART function should return a value of 3.
SELECT CAST('1999-1-1' AS datetime2) AS SelectDate
    ,DATEPART(dw, '1999-1-1') AS DayOfWeek;
GO

Reference