Wait for November, with a deadline of late in the month or early December. WFC closes in late October, and MFC in mid-November. Too much after early December and people start being on vacation or holiday-stressed, so that would push you to January.
The more general the PFC theme is, it seems, the better -- I think each variety of *FC has to find its sweet spot for the kind of theme that's just suggestive enough but not too restrictive. PFCs seem to take wide-open themes really well. -- "Late to the party" is the new "ahead of the curve" -- CRwM
With regards to theme I think people just have an easier time with landscape/abstracts. Also a wide open them like "cold" allows everyone to shoot what they want but holds the group together. -Lin Dze Arbeit Macht Frei