It is important to have a heater in the water tank to assure your horses have water available even on the coldest days.
We watch the temperature closely and blanket them when the temp is 19 degrees or sooner if they are wet from snow as they lose their body heat through their backs. We usually don't blanket before 19 degrees because we don't want them to lose their heavy hair coat. They have a barn, but they sometimes just stand outside and get wet anyway.
Sometimes if it is an intense snow storm with exceptionally cold temperatures, we just shut them in their barn stalls with fresh soft wood shavings for warm bedding. Also on cold mornings (since they drink less when it is cold), I sometimes make a tepid watery bran cereal with some oatmeal that they enjoy. It adds a little water and warmth to their bodies.
If they have shoes on make sure that the ice doesn't ball up on their feet. Just get a hoof pick and pull it out. We usually have our shoes taken off during the winter for that reason, but have had horses that need to be shod for miscellaneous reasons.
Keep a close eye for any cloudy mucous discharge that may require your vet's attention.
Make sure their hay is good quality and mold free.
Just take care of them and they will give you years of enjoyment.
Source: Experience over the years.
By Lyn from Bailey, CO