I was reading on the Johnny's Seeds website:
Winter harvest crops are planted in late summer or early fall for harvest throughout the winter. ... for harvest before and during the "Persephone Period," when day length is less than 10 hours and plant growth essentially reaches a standstill.You can look up when daylight falls below 10 hours in your town using this site: USNO Duration of Daylight Calculator. For me, its November 10.
I've tried for many years to get a cold frame full of greens to eat all winter. One winter I was successful with a good crop of spinach. Usually I plant too late; the crop holds over the winter and begins to grow again in spring for a nice spring harvest. This is good too. Some winters, its just too cold and the crops are killed.
Using my new calendar I now know that its definitely too late to plant lettuce for winter harvesting. I have some seedlings I planted several weeks ago that should be good. But maybe I could get away with sowing some spinach and arugula seeds this weekend. I'm working on putting together a cold frame or some covered hoops in my garden.