When do you transition to winter cover?

I know it's early to be thinking about this, but the Territorial fall and winter catalog arrived today. My plan is for tillage daikons to break up clay soil a bit. But the weather here (zone 9, Willamette Valley) means that I typically keep getting tomatoes, beans, etc through October. If I want to start getting some plants in the ground in April, when should I plant the cover crop. Do I need to pull my veggies earlier than usual?