I would probably do it by using a Calendar table. There are a bunch of articles around here this one: http://sqlserver2000.databases.aspfaq.com/why-should-i-consider-using-an-auxiliary-calendar-table.html

You might have to add the column for Nth WeekDay of Month

here's the function (from stackOverflow)

http://stackoverflow.com/questions/13116222/how-to-get-week-number-of-the-month-from-the-date-in-sql-server-2008