I'm in central Alberta. I start mine from seed in late February/early March under lights, then plant them out around May 20. When most of the tops have fallen over and the remaining tops are close enough to falling that I can push them over without them snapping, that's when I pull the onions and lay them on their side in the garden to dry. This year that was August 31. End of August is pretty typical. The tops are still very green at this point so I leave the onions in the garden until the tops have completely dried and can be pulled off fairly easily. That takes 2 weeks, maybe 3, depending on weather. I bring them inside in stages, that is I don't wait for them all to completely dry, I bring in dry ones when they are ready.

I don't think I've ever had them hit by frost while they lay out drying, but I don't think I'd be too concerned by a few degrees of frost. A hard frost would be another matter.

It's night now so I can't take a photo of the onions, but they would look pretty similar to this photo from 2008, which was just after I pulled the onions to dry (August 30 that year).
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Don, what about rain? Does it affect them much when they're curing like that? I just showed my husband this picture and that was his question. Right now, we don't have much rain in the forecast and i was thinking it might be a good time to harvest our onions.

Well, our onions look like that when we pull them. We couldn't leave them out because of random cold nights. Instead we hang them in small bunches in the calving barn for a month or so.Then the tops dry down and they store fine.
Thanks for sharing the picture !!
What variety do you grow ? Ours are started in the house under lights in January and I can get them out in early/mid May with mulch and row cover.
The best variety we've found is Copra, but I'd like to find an open pollinated variety in the hopes of being able to save seed. This year we are trying Clear Dawn, Yellow of Parma, Bronze d'Amposta, Brown Spanish and maybe some leftover Copra seed.
Yellow of Parma and Brown Spanish survived winter storage and are trying to make seed. One at our house and one at a friends.
Thanks for sharing : )

I've never found rain to be much of a problem because the weather usually gets much dried in September so overall they dry pretty well. The occasional rain (like the other night) just helps wash off some of the soil. This year has been very wet though and I guess if the soil stayed wet under the bulbs I would think about moving them under the deck or into the garage to finish curing. But in about 20 years of growing onions I've never had to do that.

As for varieties, I go for early storage onions. I used to grow Norland (that may be what's in the photo above), which are a medium-term storage onion and would last until the end of March or so in the basement before they would sprout. A few years ago I switched to Talon, which are a long-term storage onion and were still in decent shape when we finished the last of them in early May or so this year. Both those are hybrids though, so you couldn't save seed from them.

You never know with our Septembers, whether they'll be dry or wet, but this year looks to be fairly dry, so i think that maybe this weekend, i'll take all the onions out and leave them like that to dry. DH has often hung them up to cure in a shed, but this looks a lot easier!

Just a bit of an update... I picked the onions this year on Aug. 31 and I didn't take any pics of them right then, but they would look very much like the pic I posted above from 2008.

Here's a pic of this year's crop on Sep. 3, or 3 days after I pulled them.

And here they are on Sep. 11. Dry enough that on about half of them I could pull the dry tops off, break off loose dirt, bag 'em up, and move them into the basement for storage. The other half need to dry some more before I bring them in.

You get nice round onions. Most of ours are kind of flattened!