According to
this online biodiesel winter mix calculator, B20 should be safe down to about 4F. There are some assumptions in the calculation, so be be safe, better fill up with more petro diesel to make it B5-B10 if you expect weather cooler than 10F.
I keep a little clear jar of whatever fuel mix I have in my truck on the front porch. Glance at it as I leave on a cool morning. If the sample is clear, so is my tank. If the sample is white/wispy, better stop at the local station and throw 5gals of regular petro-diesel in before I hit the interstate.