I have washed steel and concrete structures in 27 degrees in January several years ago.
As long as the water is moving it will not freeze but you can cause ice to form on the ground.
I know guys that fleet wash until it gets to about 20 degrees, at that point they start to have ice form on the trucks.
If you are only using pressure and no chemicals, you can add some antifreeze to the tank to help keep the water from freezing.
Once you are done, that is the time to worry about your tank, machine and hoses. If you are using an open trailer everything can freeze as you are driving if it is below 32 degrees. If you are using a box truck then it can still happen, just might take a little bit longer but I have heard of guys that will keep their machines idling and circulating back into the tank, some of the guys have heaters in the box trucks to help prevent freezing.
Some of the guys use air compressors to blow out the coil, hoses, drain the water tank, drain the machine, etc.... You might not get all the water out but it might prevent serious damage if you get most out of the coil.