SQL Server has a couple of inbuilt functions to get the day of week from the given date. To get the name of the day of week, you can use DATENAME function and to get the number of the day of week, you can use DATEPART function.
Example
SELECT
DATENAME(WEEKDAY, GETDATE())
/* Result */
Monday
SELECT
DATEPART(WEEKDAY, GETDATE())
/* Result */
2
By default, SQL server always uses Sunday as the first day of the week. You can use SET DATEFIRST 1 to set Monday as the first day of the week.