DECLARE @StartDate datetime , @EndDate datetime
SELECT @StartDate =  '2/13/2000' ,
	-- Calculate 1st day of the month
	@EndDate = CAST( CAST( YEAR( GETDATE() ) AS varchar ) +
		         RIGHT( '00' + 	CAST( MONTH( GETDATE() ) AS varchar ) ,	2 ) + '01'
				 as datetime )
-- Determine 1st sunday of the month ( 7 - DATEPART...) expression
-- Add 14 days to get the 3rd sunday & do DATEDIFF
SELECT DATEDIFF( dd , @StartDate ,
	DATEADD( dd , 14 + ( 7 - DATEPART( dw, @EndDate ) ) , @EndDate ) ) AS #Days