It is my understanding long day is for northern gardens... Well, where is the line?
Onions varieties do exhibit light sensitivity in regards to when to bulb. Some will bulb with 12 hours of light and others need up to 16 hours.
You need to get the onions to a good size before the days are long enough to bulb so you have lots of leaves and good plant vigor to make a bulb.
Onions are cool weather crops. In the Southern states you can probably plant onions in January or February. You would want them to bulb before the weather got too hot, yet the days are still not that long say in April when the onions should be bulbing, so a short day onion will bulb earlier before it gets hot.
Farther North and at higher altitudes, you won't be planting that early. You will probably be planting when the southern onions are bulbing, so by the time the plants reach a respectable size and are ready to bulb the days are quite long. If you plant the short day onions here, they will bulb before the plants get sufficient size to make a big onion.
Is there a line? Probably not a distinct line, but you may take 39 degrees North Lattitude as a rough dividing line if you wish. I mention altitude because at higher altitudes, planting times are later, so you are more likely to need long day onions.
My advice is; try something and see if it works for you. It is not a big stretch to try 4 or 5 varieties of onions. Also, ask at your extension office.
My thoughts are that if you can plant onions in January or February, go for short day onions and if you must plant in March or April, go for long day onions.
Onions are hardy and can be planted early. As soon as your ground dries out enough to do it. Some frost and snow is not going to hurt them. Here in Northern Utah at 5000 feet elevation, that is usually early in April. Everything East of the Rocky Mountains is at a much lower altitude and I would think March would work for planting onions in most areas north of 39Ã‚Â° North Latitude.
I hope this helps?
Gardening at 5000 feet elevation, zone 4/5 Northern Utah, Frost free from May 25 to September 8 +/-