>>> from dateutil.rrule import *
>>> list(rrule(MONTHLY, count=3, byweekday=FR, bysetpos=-1))
[datetime.datetime(2013, 7, 26, 22, 4, 20),
datetime.datetime(2013, 8, 30, 22, 4, 20),
datetime.datetime(2013, 9, 27, 22, 4, 20)]
>>> list(rrule(MONTHLY, count=12, byweekday=FR, bysetpos=-1))[::3]
[datetime.datetime(2013, 7, 26, 22, 15, 14),
datetime.datetime(2013, 10, 25, 22, 15, 14),
datetime.datetime(2014, 1, 31, 22, 15, 14),
datetime.datetime(2014, 4, 25, 22, 15, 14)]