Franklin, great list. Here's a few more that might be worth looking at:
The season and your interests (avid outdoors people quit before summer, skier quits in late fall).
The busy time of year at work. Working all the 40 hour weeks and leaving before the 60 hour ones kick in.
When your spouse quits. If they get a huge commission, you might quit after they receive it, regardless of you not optimizing yours.
When your kids are out of school for the summer or winter holiday (or when they're NOT, depending on which you prefer).
When any fringe benefits kick in (prepaid subscription, gifts/entitlements for 15yr anniversary, etc.).
As Eric just mentioned, medical insurance is more than just that "month". If you get 90 day prescriptions, or have regular appointments, time with those (your new insurance might cause delays). If you have a large deductible, consider leaving at the beginning of your plan year, so you don't end up paying towards 2 separate deductibles.