Bulbing onions are a cool-season vegetable that falls into two categories:  long day (northern latitudes) and short day (southern latitudes).  When growing onions, they can be started with seeds, sets, or transplants. 

It is very important to plant onions early because their lifecycle is sensitive to day length.  For example, long-day onions will sprout top growth, and then will begin bulbing when there is approximately 14 hours of daylight.  If you plant early, the plants will be large by the time you reach 14 hours of daylight, and you will get larger bulbs.  If you want really large bulbs, I recommend transplants.