There are several different kinds of onions, with different planting and harvesting dates. Overwintering or short-day, onions are planted in late summer or early fall, for a late spring harvest. Long-day onions are planted in early spring, usually a couple weeks before the average date of last frost, for harvest after they bulb up in July or August. Evergreen, or day-neutral, onions are planted in early spring to early summer for harvest all summer and into fall as scallions, or green onions. Young plants of the other kinds can also be used as scallions. Onion sets are usually very small bulbs of the long-day type, and are treated accordingly, though they can be planted slightly later, if you prefer. Onions prefer a soft, sandy soil with good levels of nitrogen and potash, and barely adequate phoshate. The sulfur content of the soil partly determines how "hot" the onions get: low sulfur means a sweet onion, especially with an overwintering type, though sweeter onions are more subject to pests, and don't keep as well. They grow best in full sun, though they can take as much as a half day of shade. As with all veggies, be sure to rotate onions out with others of different plant families in different years, or assorted nasty diseases can build up in the soil.