CREATE TABLE #calendar(Dt DATETIME NOT NULL PRIMARY KEY CLUSTERED, isWeekDay  BIT);
;WITH cte AS
( 
	SELECT CAST('20130101' AS DATETIME) AS Dt
	UNION ALL 
	SELECT DATEADD(dd,1,dt) FROM cte
	WHERE Dt < '20131231'
)
INSERT INTO #calendar
SELECT
	dt,
	CASE WHEN DATEDIFF(dd,0,dt)%7 >=5 THEN 0 ELSE 1 END 
FROM 
	cte
OPTION (MAXRECURSION 0);

DECLARE @startDate DATETIME = '20130501';
DECLARE @endDate DATETIME = '20130620';

SELECT SUM(CASE WHEN isWeekday = 1 THEN 1 ELSE 0 END)
FROM #calendar 
WHERE Dt >= @startDate AND Dt < @endDate

DROP TABLE #calendar;

DECLARE @startDate DATETIME = '20130501';
DECLARE @endDate DATETIME = '20130620';
SELECT DATEDIFF(dd,@startDate,@endDate)-2*DATEDIFF(wk,@startDate,@endDate);