There is some more information on day length here: http://www.dixondalefarms.com

Dixondale sells plants, which I have had more success with than seeds.

Onion seeds are usually listed as having only one year of viability, so I imagine the germination rate in year two is fairly low. Of course low could mean as much as 50%, so if you have an excess of seeds it may be worth a try.

TomNJ

I buy onion seed in 'bulk' and keep it in the freezer, year to year. As an example, $8 in the HPS catalog will get you 2,250 seeds for 'Candy". While the viability may decrease year to year, I haven't noticed it that much, now on year 3 for the same seed packet. We'll see how year 4 goes......

Start the seeds in early Feb, they are pencil-sized+ in April/May when I set them out. The advantages are - far less transplant shock, they don't skip a beat when they're planted. The roots are significantly larger and more developed.

In the Culture blurb on the onion page, they state "develop and promote the best new hybrids for Northeastern growers." Based on that, I would guess most of the onions they carry, especially the hybrids, are long day onions. You could also check out the Seminis, Takii, Crookham, and Nunhem websites (those companies are the major breeders that produce the hybrids Stokes sell) to see what they describe them as (example, Alpine, Norstar, Trailblazer are long day according to Takii).

I think Cyrczch has hit the nail on the head. Check what the breeders say about their varieties.
The only other thing I would add, is that if Stokes doesn't list enough info for you, don't buy from Stokes. I personally haven't found their quality to be very good.

Ditto on onion seed storage in the fridge. Two years ago I got a great deal on a gallon bucket of pelleted onion seed from a large seed company. It was listed as 2002 seed when they pulled it from storage 2 years ago and sold it to me for cheap. I've planted thousands of seeds and still get 40-50% germ with good vigor.

Concur: Stokes only vends long day onions which is to be expected since it is basicly a Canadien Company. Long Day onions are grown in the deep south (Z8-9). Intermediates like Candy, Red Candy Apple, Superstar,Cimarron are pretty easily recognisable. In you area short day varieties are probably best, but the intermediates should do allright.
Most seeds will keep for a long time in a freezer. Under ordinary storage onion seeds are very short lived.

Uh, farmerdill, I think you know what you're saying but saying it wrong. Long day are northern varieties, short day onions are southern.
At a latitude of 41 the OP should be planting LONG DAY onions or (as you stated), intermediate.
I think you just mixed up the wording but have the right idea.

Right: I am getting old. Long day onions for the north. Short day onions are grown in the south in winter for harvest in April and early May.
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