Monday Date Calculation

-- SQL user-defined function
-- SQL scalar function – UDF
CREATE FUNCTION fnMondayDate (
	@Year INT,
	@Month INT,
	@MondayOrdinal INT
	)
RETURNS DATETIME
AS
BEGIN
	DECLARE @FirstDayOfMonth CHAR(10),
		@SeedDate CHAR(10)

	SET @FirstDayOfMonth = convert(VARCHAR, @Year) + '-' + convert(VARCHAR, @Month) + '-01'
	SET @SeedDate = '1900-01-01'

	RETURN DATEADD(DD, DATEDIFF(DD, @SeedDate, DATEADD(DD, (@MondayOrdinal * 7) - 1, @FirstDayOfMonth)) / 7 * 7, @SeedDate)
END
GO

-- Test Datetime UDF
-- Third Monday in Feb, 2015
SELECT dbo.fnMondayDate(2016, 2, 3)

-- 2015-02-16 00:00:00.000
-- First Monday of current month
SELECT dbo.fnMondayDate(Year(getdate()), Month(getdate()), 1)
-- 2009-02-02 00:00:00.000

Source: http://anubhavg.wordpress.com/2009/06/11/how-to-format-datetime-date-in-sql-server-2005/