Good question. I was wondering the same thing. I live in south central PA on the dividing line between 6A and 6B. I actually considered sowing leaf lettuce this weekend.

I sowed lettuce and spinach today. My brother in West Chester did his lettuce and onions last weekend. But my lettuce was seeds. Maybe plants should be a little later because of foliage.

I planted lettuce seed, spinach seed and peas today.

We planted lettuce today in Lanc County too....

I have fairly large size spinach already growing from seeds in the ground left over from last year. So far, I've planted lettuce, argula, broccoli rabe, onions, potatoes, peas and Swiss chard.

I direct-sowed lettuce about two weeks ago and it's coming up now. I've never started lettuce indoors, not sure when you can put it out.