Cron Expression - Starting at specific day/time and repeating till end of year

Cron Expression - Starting at specific day/time and repeating till end of
year

I want to write a cron expression which does the following -
Starting every last Sunday of September at 9:00 am and repeating every 3
weeks till end of the year.
I have come up with
0 0 9 ? 9 6L
How can i make this repeat every two weeks till end of the year.
Thanks in advance