Fireworks only appear once the in-game clock reaches New Year's or the fourth of July (since the game originated in the US) or right after a city is established.
The fireworks display will last for about a minute before it will conclude. It is purely for entertainment value, and it will not affect the city whatsoever.
- It does not matter whether it is light or dark, the fireworks will appear, even if you set it to be broad daylight.
- There is a glitch where occasionally the fireworks do not go off on the given dates. This usually happens when the game is at maximum speed. There has been no official patch that fixes this problem.
- The fireworks will go off automatically; there is no possible way of turning off the fireworks, nor is it possible to set them off yourself without external modifications.