Answer: Rotating vegetable crops means simply that you'll want to plant different veggies in your garden beds in different places. For instance, plant lettuce where you've grown tomatoes before and corn where you had carrots. Moving the crops around avoids depleting the soil of certain nutrients, plus it helps keep insect and disease problems under control. The way I remember how to rotate is to plant an underground veggie such as carrots then the next year planting an above-ground veggie such as corn, followed by a vining type veggie such as tomato or cucumber.
Onions like cool weather so you can plant yours now. Give the plants 6" on all sides so they're not crowded as the mature. As for your veggie seeds, I'd plant them the last week of April and invest in some row covers. These are made of lightweight polyspun material and can be draped over the rows of veggies to help protect them from frost if the need arises.
Best wishes with your garden!
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