Now. Start now.

A couple of reasons...first, you need to get out there and start interviewing the bike shops. Can you work with them? Will they work with you? Do they treat you like a wise consumer or...sigh...like a dumb girl? They're not as busy right now so they'll have time to talk.

Get the bikes on the trainer and see how they feel. Get a fit done so that you know what bikes you should be looking at. You can start to narrow down the field (and the field is big).

And, you can still test ride in winter. I live in Wisconsin--we're a little warmer than you, but when the roads clear a little you can get a test ride in. I won't ride in snow (no sense of balance) and I won't ride in bitter cold, but in between the big snow falls you could get in a short test ride to start to feel the differences between frame geometries and materials.

Perhaps you won't want to make a final decision until spring, but having the field winnowed down to one or two would mean you'd get that bike sooner.