There is no single answer to this: AFAIK there is no standard for week-of-month which is universally accepted - in fact the definition can vary even within a single country, unlike week-of-year which is covered by ISO8601. If you look
here[
^] you will see what complications WOY introduces, and you should realize that these also apply to WOM.
You will have to look at the year and month and work out what week day 1 is in first, then work forward to find out which range of dates ar in the required on. Be careful though - if your rules are anything like WOY, some of your days may be in last month, or next month because weeks don't start on the first of the month.
This may help:
http://stackoverflow.com/questions/2136487/calculate-week-of-month-in-net[
^]