Liz Buchanan BVSc MRCVS
The main thing that you should give a frightened animal is somewhere where they feel relatively safe, to hide. If thats the bathroom then it's the bathroom, as long as they are safe (check for hazardous chemicald etc if its not a room they usually use and leave any toilet lids closed). There may be 'calming' products available from your vet that can be put in the 'safe' spot for extra reassurance (the pet should be accostomed to this before the big night); the smell of relaxation (perhaps a coat you have worn for a walk, or bedding of theirs) may help. The idea is to create a bolt hole, a place of relative safety and not a prison - pets should be able to choose to use it or not.
In the past. Vets used to sedate some animals at certain times of year; the evidence shows that some of these products are less useful than it used to be thought, somd actually immobilising the pet without necessarily removing the fear sensation. Your vet should discuss this with you.
There is a discussion going into much more detail about anxiety in the blog. I think that if you type the word 'firework' into the Blog search-bar, you should be able to access it.
I hope that something there helps.