Victor Ograygor said:
Who is throwing it? Why? How fantastical is your world?
I imagine that it will be thrown by wizard's to get together and meet. The main attraction for wizards will be a chance to get together and talk and exchange knowledge and spells. This will probably take place inside the hosting wizard's (or cities) buildings away from the public and for wizards only. Even then, some of the major talks will probably be for invited wizards only while other talks will be for all wizards or even including the public. mages will be given a place to meet, talk, discuss business, and copy spells without interuptions from the public or each other.
Outside, there will be others looking to do business with the mages. The mages themselves might have tents set up to do business. They can sell potions and scrolls, or even take commisions for magic items. Some might be looking for apprentices and reviewing children for those of promising above average intellects (or of high birth and money whose parents can pay). it will also be a prime time and place for wizards who need it to find training. Important wizards will need their meat shield so fighters and the like might be looking for work.
The fair grounds will have a lot of stuff that has no game effects. Spell comonents, clothing, staves, and other items of all types will be for sale. If allowed, people will want to wander around just to look. With the public will come the food vendors and other vendors who will see the large population as a chance to sell their non-mage related wares too. Craftsmen will probably also be there to show off their masterwork items in hopes to get contracts to make things that will later become magic items. In the crowd will be preformers and entertainers trying to make money. There might even be a stage set up for them.
In the inns and taverns of the city, business will be in the works. Adventurer parties might arrive to buy or sell magic items, let their wizards join in, or even to look for a new wizard. Wizards will be meeting and discussing plans. Other important people will be there to do business with the wizards. Any groups of wizards might arrive and take care of business if they do not meet very often.
If you have a fantastical world, anything goes. Wizards and illutionists showing off spells. People selling exotic animals and monsters both live and dead. Wizards could be walking around wth golems, exotic familiars, and other displays of power. Some intelligent monsters may show up, especially if they have wizard like abilities. Depending on how accomodating the city or kingdom is, they might walk openly, in disguise, or suffice to have agents do their bidding. Displays of power, duels, or magical games might be played for both prestige of the wizards as well as entertainment of the crowd. Some duels might settle old vendettas or start new ones.