Onions are day length sensitive and bulb when the days are of the right length. They are also a cool weather plant so need to be planted early to grow while it is cool. In the South onions can likely be planted in December and they will be old enough to bulb while the days are still short. Hence Short Day Onions for the south. In Northern latitudes planting time is later when the days are longer, hence Long Day Onions for the North.
If I plant seed here the best I can hope for are some the size of golf balls. (or plenty of green onions) If I plant sets or small plants, I can get some nice large bulbs.
Some types are said to be day neutral.