Decide early: stabled in, or out in the field
There is no universal right answer, only the right answer for this horse. One who is settled in his stable routine, with company he can see, usually does better in, with deep bedding, a full haynet and a radio playing on the yard. One who box walks, sweats or injures himself when shut in is often safer out in a small, familiar paddock with a calm companion, where he can move his feet. Decide in the weeks before, not on the day. If you are genuinely unsure, talk it through with your vet, who knows the horse's history; genuinely anxious horses sometimes need a planned conversation about calming support well ahead of the night, and that decision belongs with the vet rather than in a last-minute grab for something unproven.
Teach the sound before the night
Fireworks are frightening partly because they arrive from nowhere. In the weeks before, play recorded firework sounds at low volume while the horse is eating — the pairing of bangs with his feed is the whole trick — and raise the volume a little every few days, dropping back a level whenever he stops eating or starts staring. Horses whose yards sit near regular displays often habituate surprisingly well when the sound ladder is unhurried. The recordings also help you judge your own horse early: one who settles back to his hay at moderate volume with you nearby will probably cope on the night, while one who is still wound up tells you to strengthen the rest of the plan.
The yard safety sweep
On the day, walk every boundary the horse could reach: fence rails and wire sound and properly tensioned, gates shutting and bolting as they should, water troughs full, and nothing left in the stable or paddock that he could crash into or tangle in — move the wheelbarrow, the jump wings and the fence reel out of harm's way. In the stable, check that the door and its bolts hold firmly, bed down deeply so a scrabbling horse has grip and padding, and hang more hay than usual, because a horse with his head down eating is a horse whose heart rate is dropping. If neighbours are planning displays, ask about likely start and finish times; knowing the shape of the evening shapes when you check.
On the night itself
Feed and settle the horse before dark, at the normal time and in the normal order, because routine is the message that nothing is wrong. Stay nearby if you can, or arrange for someone experienced to be on the yard, and time your checks between displays rather than walking into a field full of lit-up horses mid-bang. Speak and move as though nothing is happening, because your tension is data to a horse. Afterwards, check legs, eyes and appetite before you leave, and look again at first light: cuts, a swollen joint, scuff marks on the stable wall, or a horse off his feed and tucked up after a stressful night all warrant a call to your vet, and laboured breathing or colic signs such as pawing, rolling and looking at his flank warrant it immediately. And should a frightened horse break out in the dark, his KinsLe profile can carry a map display at no cost, with a direct route back to you for a finder who meets him — the Lost Pet Alert System in KinsLe gets that sighting to you without a hunt for your number.