You may want to find for example the date of the 4th Saturday in each month for a given year. This function came out of answering the question here: http://stackoverflow.com/questions/33694768/how-to-get-list-of-2nd-and-4th-saturday-dates-in-sql-server.
I’ve created it as a Table Valued Function so you can bind it into any query you wish.
Tony created a Table-Valued Function, which is handy but leads me to the classic User-Defined Function reminder: they tend to cause performance problems. One alternative is a dedicated date table with attributes like day of week and nth day of month.