At what temperature does water freeze, in Celsius?

Zero, because the scale was built around it: Anders Celsius fixed his two reference points at the freezing and boiling of water, then divided the gap into a hundred. He put boiling at zero and freezing at 100, and the scale was flipped to the version we use shortly after his death in 1744.

The behaviour at that point is one of the strangest things water does. Almost every substance contracts as it solidifies and its solid sinks in its own liquid. Water expands by about nine per cent when it freezes, so ice floats. If it did not, lakes and seas would freeze solid from the bottom up and stay that way, and the argument that life on this planet depends on that quirk is not much of a stretch.

Nor does water reliably freeze at zero. Freezing needs somewhere to start - a speck of dust, a scratch, an existing crystal - and very pure water in a very smooth container can be chilled to around minus forty before it gives in. Clouds are full of supercooled droplets doing exactly this, which is why aircraft ice up: the water is waiting for a surface, and the wing provides one.