The Fair is a building.
This building is an improvement of the Great Market. It therefore requires the latter to be built.
To construct a Fair, it is necessary that none of the adjoining provinces AND the provinces adjoining to these constructs a Fair itself. If one of your neighbours (or neighbours of your neighbours) finishes the Fair before you, while you have already started, your work is cancelled and you lose all invested means!
This building requires 36500 man-days
The construction of the building requires 5 political points, 12,000 gold, 40 wood, 50 stones and 20 iron.
This building raises the taxes in the province by 0.6 points, including the 0.2 bonus from the Great Market
It also raises the tax for goods travelling through the province (trade between other provinces) by 10 points.