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 import datetime

 def get_nth_weekday(year,month,weekday,n):
    """
    weekday=0 is Monday
    n=1 means 1st weekday
    """
    for day in [1,2,3,4,5,6,7]:
        if datetime.date(year,month,day).weekday()==weekday:
            #print datetime.date(year,month,day)
            break
    return datetime.date(year,month,day)+datetime.timedelta((n-1)*7)

 今月の第3月曜日は
 >>>print get_nth_weekday(datetime.date.today().year,datetime.date.today().month,0,3)
 2007-10-15

この手は使えないか?

http://www.python.jp/doc/release/lib/module-calendar.html