The following Microsoft SQL Server T-SQL scalar-valued function returns Monday's date for any input date:

-- SQL Server user-defined function - UDF - SVF

USE AdventureWorks2008;

GO

CREATE FUNCTION fnMondayOfWeek

               (@AnyDate DATETIME)

RETURNS DATETIME

AS

  BEGIN

    RETURN dateadd(yyyy,datepart(yyyy,dateadd(weekday,1 -

           datepart(weekday,@AnyDate),@AnyDate)) - 1900,

                   0) + dateadd(dy,datepart(dy,dateadd(weekday,1 -

                   datepart(weekday,@AnyDate),@AnyDate)), 0)

  END

GO

SELECT dbo.fnMondayOfWeek('2016-10-23T00:00:00.000')

GO

-- 2016-10-24 00:00:00.000