Placing concrete in freezing conditions can be done, as long as there are procedures in place to deal with the conditions. These procedures are always reviewed by the Project Structural Engineer. The suspended slab forms require heating from below to ensure fresh concrete doesn't freeze from below. Also, the slab perimeter should have wind breaks installed to keep the surface from freezing. In milder cold weather, walls and columns might only need to be wrapped in insulated forms. In sustained freezing weather, the forms would need to be constructed with insulation panels. And the concrete is usually a special mix design that is suited to cold weather placement. It is usually cheaper to continue with construction than to shut down a site for 3 months. Also, the structure is the best thing to do in cold weather in regards to the overall construction process. The last thing you want to do in freezing weather is envelope work. All hand work that requires perfection or else it leaks.
My two cents.