When you are heading out together, remember to :
Keep an unobtrusive eye on how your partner is coping, especially if they're consuming alcohol.
Be aware of the signs that indicate that your partner is either not coping or is nearing capacity.
Make sure you remember to take any food your partner might want (as they may be too anxious to remember this themselves).
Organise return transport berfore the event if neither of you are driving. Taxis get booked up quickly at this time of year.
Help you partner to suss out any areas where they can take a break from the busyness as soon after you arrive as possible. Anywhere they can escape the full-on environment even for a short time, can provide welcome relief for when the noise, people, lights, etc get too much.
Make sure that your partner got their ear buds / noise cancelling headphones / flare calmers wih them.
Advocate for your partner if and when needed. Agree in advance whether, if your partner wants to leave sooner than you do, will you see them safely into their car or a taxi and stay on or will you leave with them?