FORMART function allows you to extract DAY part from a datetime value , also it is very useful to represent extracted part in different forms..
For example, the below code snippets explains you to extract DAY part from a DATETIME value in Full Date, only day in digits, simple day name, full day name forms...
SELECT FORMAT ( GETDATE(), 'd', 'en-US' ) AS US_Result; -- 3/20/2015
SELECT FORMAT ( GETDATE(), 'dd', 'en-US' ) AS US_Result; -- 20
SELECT FORMAT ( GETDATE(), 'ddd', 'en-US' ) AS US_Result; -- Fri
SELECT FORMAT ( GETDATE(), 'dddd', 'en-US' ) AS US_Result; --Friday