United Arab Emirates has 2 festivals in this guide, spread across 1 city. They run in February and December, and festival-day temperatures range from 26°C (79°F) to 26°C (79°F), with nights down to 17°C (63°F). That range is the reason there is no single "United Arab Emirates festival outfit". The answer changes by month and by site.
Conditions are moderate across the board, so the day-to-night temperature drop is the thing to plan for rather than the weather itself.
The ground matters as much as the sky. You will find arena grounds and city streets, which puts broken-in trainers or a low boot at the top of the list for most of these events. None of them have camping on site, so you are travelling in and out each day and the outfit has to survive the journey at both ends.
Stylistically the crowd here leans streetwear, rave, glam and edm. UNTOLD Dubai is the largest of them at around 150,000 people, and it sets the tone the smaller ones react to. None of that is a dress code. It is what you will see, which is worth knowing if you would rather blend in or deliberately not.