A: Hi - Here's a great answer to your question (from sanctuarygardener.wordpress.com):

"Last fall, I decided to try growing onions from seed. I figured I’d plant some seed in the fall and onion sets (baby onion plants) in the late winter. All excited, I ordered a couple packets of onion seed. When they arrived, I noticed the packets stated they were long-day onions. Curious, I researched onions online – and found out I bought the wrong type of onion seed for my latitude.

Unlike most plants, which are labeled with proper hardiness zone, onions are labeled by the amount of daylight hours needed to bulb. Short-day onions ..... need 14-16 hour days.

If you live up north (Zones 1-6), you should grow long-day onions. The great number of short, winter days will give your onions lots of top growth before bulbing. This will help the plant grow large bulbs. The onions will be hard, have a pungent flavor, and store well. If you were to plant short-day onions, the plants would bulb early and be very small.

If you live in the south (Zones 7-10), as I do, you should grow short-day onions. We have less variation in day length between seasons here. (I always tell my family in New England that we have only two seasons here: summer and not-summer.) The onions will have a high water content and won’t store well. So, here, we have to refrigerate them or pickle them. If I would have planted those long-day onion seeds, they wouldn’t have received enough day length to trigger bulbing."

For more information please go here: sanctuarygardener.wordpress.com/2013/02/18/long-day-vs-short-day-onions