The picture in your head is wrong
When someone asks me if windows can be replaced in winter, they are usually imagining the whole side of the house open to the weather while a crew works through it.
Not quite.
A replacement is a one-opening-at-a-time job. The old window comes out of a hole that is exactly window-sized, the new one goes into that same hole, and it is sealed before anybody touches the next one. The opening is uncovered for a stretch of the working day — not for days, and never the whole house at once.
That single fact takes most of the fear out of the question. The room gets cold. You'll feel it, and you'll want the door to that room shut. The house does not.
What genuinely changes in the cold, and it isn't the window
The window itself does not care. It's a rigid frame and a sealed glass unit; it is going to spend the next few decades outside in this climate anyway. If it couldn't handle a Calgary January, it would be the wrong window.
What cares is everything used to fit it. Expanding foam, sealants, adhesive tapes, membranes: each of those products has a working temperature range set by whoever manufactured it, and that range is printed on the packaging. Some are formulated for cold-weather application. Some are not, and applying them below their range means they cure differently than the manufacturer intended.
This is the whole ballgame, and it is completely checkable. Ask the crew which foam and which sealant they use in winter, then look up that specific product's stated range. Any crew that genuinely works through prairie winters will answer that in a sentence, because they had to solve it years ago.
Vague answer, no product name? That tells you something too.
The part nobody advertises: winter shows you the truth
Here's the honest advantage of a cold-weather installation, and it works in your favour, not mine.
A bad seal is invisible in summer. You could have a gap in the insulation around the frame, air tracking through a corner, a perimeter that was never properly filled, and in July you would never know. Everything feels fine when it's warm on both sides of the glass.
In January that same gap is a cold ribbon you can feel with the back of your hand.
So when the crew is finished and the window is in, wait for a properly cold day and go around the whole perimeter slowly: corners first, then along the bottom, then the sides. You are feeling for moving air. If you find it, say so straight away, while the job is fresh and everyone still remembers that window. That check is worth more than any warranty clause, and it is only available to you in the cold half of the year.
Why winter scheduling is often better
Summer is when everybody decides to do their windows. Crews are booked out, dates slip, and the good installers are the busiest of all.
In winter the queue is shorter. Lead times come down, you have a real chance of getting the experienced crew rather than whoever is free, and the date you agree to is more likely to be the date that happens.
If a company offers a winter discount, fine — take it. But that's not the reason to book in the off-season. Getting the attention of people who aren't racing the calendar is.
What actually gets harder, honestly
I'd rather tell you this part than have you discover it on the day.
Daylight is short, so the working day is short, and a job can stretch over more days than the same job would in June. Snow and ice have to be cleared from the working area outside before anyone can move safely with glass in their hands. And some exterior finishing work, certain caulking, cladding details, paint, genuinely should wait for a warmer stretch rather than being forced now.
That last one is not a problem, it's a scheduling item. The problem is a company that doesn't mention it. Ask directly: which parts of this job get finished in spring, and is that written into the contract? A straight answer means you're dealing with someone who has done this before.
What to ask before you book a winter installation
Short list. Any crew that works through winter will have these answers ready.
Which foam and sealant do you use below freezing, and what temperature range does the manufacturer state for them? Product names, not reassurance.
How many openings will be open at once? The answer you want is one, or one per crew working independently.
Which parts of the job wait until spring, and is that in writing? Exterior finishing usually does. It should be on the contract, not in a conversation.
What happens if it's a genuinely brutal day? There is a point where a responsible crew reschedules. Find out where their line is before you're standing in it.
How do I check your work afterwards? If they walk you around the perimeter themselves and tell you to feel for drafts, that's a good sign about the whole company.
When I'd tell you to wait
If your windows are simply old-looking but sound, they close, they lock, the glass is intact, nothing is rotten, then waiting for spring costs you nothing and is easier on everyone. There is no urgency and I'm not going to invent one.
If the glass is cracked, the frame has gone soft, the sash won't close properly, or you can feel the outside coming in when you walk past — that is different. Waiting until May means paying to heat outdoor air through the coldest months of the year, and putting up with the room you already avoid sitting in.
The real comparison was never winter versus summer. It's one more winter with the window you have, against the one you'd put in its place.
Why I'd rather you asked all of this
Every question above is one you can put to me or to anybody else quoting your house, and the answers are checkable against a product label and a contract.
That's deliberate. I would rather lose a job to someone who gave you better answers than win one because you didn't know what to ask.