Yes, but it takes precautions. Concrete can be poured when outdoor temperatures are below freezing, but the concrete, subgrade, forms, and curing environment have to be kept warm enough to prevent early-age freezing. Use a cold-weather mix and protect the pour until it has gained enough strength.
One of the ingredients in concrete mix is water. If that water freezes before the slab has gained strength, it can ruin the pour. There are many ways to make a winter pour safe and doable, and most of them come down to what you do before the trucks show up.
Concrete temperature is not air temperature
There is sometimes a misconception that concrete temperature and air temperature would be the same. That is not true. Concrete forms chemical bonds which produce heat, placing the temperature above air temps.
Commonly contractors tend to start taking cold-weather precautions around 40F. That is also where ACI 306, the industry guide for cold weather concreting, draws its line. It can vary depending on the mix and other site conditions.
What matters more than the air temp is how warm the concrete is when it goes down. Thin work has to be warmer. A 4" slab has a lot of surface for its volume and gives up heat fast, so you want it coming off the truck around 55F. A mass footing holds its own heat and can go down cooler than that.
The number that matters: 500 psi
Concrete needs to reach about 500 psi before a freeze can't hurt it. At that point enough of the mix water has been used up by hydration that what is left won't do damage when it freezes.
Seven things to get right
Check the full weather
Review not only the daytime temps, but the overnight lows, wind, and the forecast for the days after. If conditions are unfavorable, extreme cold and wind, it may be best to delay the pour. Wind pulls heat off a slab faster than cold air alone does.
Thaw and prepare the subgrade
Remove snow, ice, and frost. Never place concrete on frozen ground, which can draw heat from the mix and settle unevenly after thawing.
Warm forms, reinforcement, and embedded items
Cold contact surfaces draw heat from the concrete. If possible, heating the forms, rebar, and any embedded items helps the concrete cure correctly and prevents inconsistencies in the finished product.
Order the right cold-weather mix
Redimix plants have great resources and people to work with, and they can help you decide on the correct mix. You can add a lot of things to the mix, but with each addition comes an increased cost. You do not want to add too much where you lose money on your job, but you want to add enough that you don't ruin the job.
Things you can add include heated mixing water or aggregates, high-early-strength cement, additional cement, and accelerating admixtures.
Two things worth asking the plant about:
Which accelerator. Accelerators that use calcium chloride are cheaper, but chloride corrodes rebar over time. On reinforced work, ask for a non-chloride accelerator. On a plain sidewalk it matters a lot less.
Air entrainment. If the slab is going to see freeze-thaw cycles once it is in service, and in most of the upper Midwest it will, it needs entrained air. This has nothing to do with getting through pour day and everything to do with whether the slab is still good in ten years.
Place and finish efficiently
Placing and finishing efficiently is vital in winter concrete. Having everything prepped before the trucks arrive lets you minimize delays. Once the concrete is placed you want to avoid finishing bleed water into the surface, and to cover the concrete promptly.
If the concrete requires troweling, the contractor may add accelerator beforehand so the process is sped up. Cold concrete sets slow, and waiting on a slab in the dark at 25 degrees is how finishers end up rushing the wrong step.
Insulate or heat the concrete
Many contractors choose to use insulated curing blankets. They are essentially an insulated tarp, often made of polyethylene layers and foam cores, and they trap the concrete's natural heat instead of letting it escape into the air. Some contractors build heated enclosures. These are effective but expensive.
Edges, corners, and thin sections may require more insulation and attention because they lose heat the fastest.
Know when to pull the blankets
This is the step most people skip. Leave the protection on until the concrete has reached that 500 psi, which is roughly 24 hours at 50F or 48 at 40F. Pulling covers early to get them onto the next job is how a slab that looked fine on Tuesday scales and spalls by spring.
The short version
You can pour all winter. What you can't do is pour the way you would in July and hope it works out. Check the whole forecast, not just the high. Get the frost out of the ground. Order the mix for the conditions instead of the calendar. Protect it until it has strength, and take the protection off gently.
None of this is complicated. It is just a longer list than a summer pour, and every item on it is cheaper than tearing out a slab in April.
Bidding a winter pour? Run the yards in ConCal, or read more on concrete curing time and choosing a PSI.