Find out when the fireworks are coming
Organised displays advertise their dates, and those are the easy ones to plan around. The harder risk is the private party two fields away, so in firework season make a point of asking neighbours, checking community boards and listening for the first exploratory bangs in the weekends running up to the big dates. Plan for the whole season rather than a single night: horses are unsettled as much by the unpredictability — three quiet evenings, then an hour of war — as by any one display. Once you know the likely nights, arrange your week so the horses' exercise, feeding and your own presence are already sorted before the first rocket goes up.
Stable early, and keep everything deliberately ordinary
Bring horses in well before dusk, into their usual boxes with their usual neighbours, because familiarity is the strongest sedative you own. Give each a full hay net to work at, leave a radio playing at everyday volume so the bangs land on top of background noise instead of silence, and keep the yard lights on to take the edge off the flashes. Feed at the normal time, muck out at the normal time, use the normal rugs. Resist every urge to make the night special with fussing and repeated checking through the door, which tells a horse that his human is worried too. Some horses genuinely do better turned out with their herd in a large, distant field; if that is your horse's known preference, honour it — but choose one plan per horse in advance, not in a panic at the first bang.
Secure the yard against a panic escape
Earlier that day, walk the perimeter fence of every field that might hold a horse, check stable doors and bolts including the kick bolts at the bottom, and clear the boxes of anything a scrambling horse could catch himself on — hanging haynets tied too low, equipment stored in corners, protruding hooks. Shut and latch yard gates before dark so that the worst case is a horse loose on the yard rather than a horse loose on the road. Think, too, about the horse who panics in the box: deep bedding, a clear space to turn, and nothing at head height he can strike against are worth ten minutes of your afternoon.
During the fireworks, be present and be boring
If you can be on the yard, be there: move about the block in your normal rhythm, speak in your normal voice, top up water, and let the horses see that the person they trust is entirely unmoved. Watch from the door rather than entering the box of a horse who is throwing himself about, because a panicking horse does not know you are in there and a confined space with a tonne of frightened animal has no safe corner. Note which horses are sweating, trembling or refusing hay, and keep a mental list for the checks that follow once the noise dies down.
After the last bang, check everyone properly
When it is over, go box to box and field to field: look for cuts and swellings from scrambling, cast rugs, broken boards, and horses who are still distressed and not settling once the world is quiet again. A horse with an injury, or one whose distress is not easing, gets a vet call that night rather than a wait-until-morning. If the worst happened and a horse broke out and left the property, minutes matter: shut the yard, search the immediate lanes with help and torches, and get the KinsLe Lost Pet Alert System alerting nearby members with his description while the search is young, because a frightened horse found quickly by many eyes is a horse brought home before he finds a road.