Diwali's celebrated by Hindus, but also Sikhs, Jains, and some Buddhists and Muslims. That's a huge number of people in itself, so major variations occur.
There's different norms for each, including within sects, according not only to religion, but also to if they are from South, North, West, or East.
When people fast, and what they eat and when, varies, but for a majority it would be normal to abstain from meat and alcohol as the festival represents in different ways for different faiths the same basic ideas of victory of light over darkness, knowledge over ignorance, and good over evil.
Hindu scriptures have the concept of ahimsa, which is non-violence and compassion towards all beings. How thoroughly it's practiced varies hugely.
The vast majority of Jains are vegetarian, about two thirds of practicing Sikhs, and a higher percentage than that of N.Indian Hindus, but only around a third of S.Indian Hindus, and about a fifth of NE. and E. Indian Hindus. What caste someone is may also influence things, and where in the world they are living.
Diwalli gets mainly celebrated over five days, but again there are variations all over the world.
Food's a big part of Diwali celebrations but if for instance you where at an event held by Tamils, then chicken and goat would be ritually normal, followed by starting a fast.
I've been to many a good Sikh event where alcohol flowed freely despite it being officially banned. People are diverse basically, and like minds tend to flock together. (I love alcohol, but am life long vegetarian and don't drink anything involving isinglass, gelatin or cochineal. My Sikh friends observe that too, but not the no drinking.)
If you're in politics it's common sense to follow whatever is least likely to cause offense to the majority, and the majority would be abstaining. HTH.
Enjoy the party!