Just out of interest, This is possibly a lot more difficult to understand, but shorter. (the same code will detect the nth day you specify. I just stripped it down a bit to do the first monday! See https://www.simple-talk.com/sql/learn-sql-server/robyn-pages-sql-server-datetime-workbench/ )

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IF OBJECT_ID (N'firstMondayOfMonth') IS NOT NULL

  DROP FUNCTION firstMondayOfMonth

GO

CREATE FUNCTION firstMondayOfMonth (@TheYear CHAR(4), @TheMonth CHAR(3))

RETURNS DATETIME

WITH EXECUTE AS CALLER

AS

  BEGIN

  RETURN DATEADD(MONTH, DATEDIFF(MONTH, 0, CONVERT(date,'1 '+@TheMonth+' '+@TheYear,113)), 0)+6

        -(DATEPART (Weekday, DATEADD(MONTH, DATEDIFF(MONTH, 0, CONVERT(date,'1 '+@TheMonth+' '+@TheYear,113)), 0))

          +@@DateFirst+4)%7

  END

GO

IF (

  SELECT dbo.FirstMondayOfMonth ('2012','jun'))<> '2012-06-04'

  RAISERROR('''firstMondayOfMonth'' stopped working (1)',16,1)

IF (

  SELECT dbo.FirstMondayOfMonth ('2014','Sep'))<> '2014-09-01'

  RAISERROR('''firstMondayOfMonth'' stopped working (2)',16,1)

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