It will be -15F for a high this coming Monday/Tuesday. Honestly, single digits don't sound that bad. ;)
One note I'd like to make, if it hasn't been made already, is about hand warmers. Do NOT use foot warmers for your hands. Your hands are much more sensitive and foot warmers get VERY hot, hence why they are sold for your feet. Also, do not throw active warmers away. They will sit in your garbage and sizzle and get very hot. Not sure if they would start on fire, but I wouldn't want to find out. Instead, you can take them and seal them in a ziplock back. When there is no air to them, they stop working. Then, you can take and re-activate them by bringing them back out of the bag.
If you are going to blanket your horse, make sure you have the right blanket for the job. A small sheet will just smooth down a horse's natural insulation. Horses fluff their hair out to insulate themselves. If you blanket them, get them good protection.
It is important to keep them DRY and OUT OF THE WIND. If you work them, especially if it is below zero, don't get them sopping wet. After working if they are at all wet or steamy, give them time to 'cool down' before turning them out in the elements. If their hair is smoothed down, I generally try floofing it up for them. Letting them have a roll in the arena will usually get them to shake and fluff up.
For the humans; don't run out in jeans and a sweatshirt if it is below zero! (Maybe this goes without saying, but for those who are used to cold it is easy to think, "I'll just quick go and do this...") Even if you only have to toss a few flakes out or check, take the time to cozy up. Long johns are your best friend. Wear them under jeans, then if it is extra cold, under snow pants. Sweatshirt underneath an insulated, wind breaking jacket. A hat to top it off, and if lots of wind or excessive cold, a scarf or ski mask to cover your mouth/nose. Buy a nice pair of gloves - it is worth it! If you are doing anything with horses, you probably don't want big bulky ones. Find slim-fingered ones that still give you some dexterity.
Make sure your horses have access to water frequently. We have heated automatic waters. (Before you think this is all peaches and ice cream, they usually get to be a pain when it is single digits and below zero, so quite frequently here.) Some horses you might want to keep an eye on. Horses that are old, or colic-prone should get some extra care. Even on water with heaters, ice will still have to be broken. Morning and night if it is really cold.
Having seen some very suckish weather, I would not even fret about anything over 15F. If you are not used to it, it'll feel freezing cold, but so long as you are bundled up it is fine.