Would you rather set off fireworks at night or in the daytime? At night, of course! That's when the beauty of the pyrotechnics can be seen clearly against the dark sky. If you were to set them off in the day, you'd barely even be able to make them out in the bright light!
The burning houses of Ray Bradbury's Fahrenheit 451 are, in many ways, analogous to fireworks we put on for show. They are meant to be a big and grand spectacle. They are meant to be entertainment for everyone who is watching, a beautiful and impressive show. The people watching fireworks and burning houses are respectively meant to be amazed and wowed by the visual splendor of the event. Montag and everyone else in his community knows that bright pyrotechnics are always prettier and brighter at night time. So, we decide to light off fireworks in the night, and the neighbors of the Fahrenheit 451 world decide to sound the alarms for the firemen at night.
Of course, the analogy ends there because the burning houses also act as a dangerous political message to people who have books in their houses. Fireworks are usually not a political message nowadays.
At night, the fires are also more easily seen by the people that need to be warned. Those secret readers and idea-havers and scholars and thinkers who are hoarding illegal words in their houses... they are the ones who are meant to see the bright spectacle as a warning... get rid of those books, or we'll burn down YOUR house too! It's a constant reminder of the risks they are taking in order to perpetuate knowledge, imagination, and free thinking. It also shows how ingrained the idea has become, that books are bad. Neighbors will always turn people in by sounding the alarm.
The fire alarms always come at night because a fire burning at night gets more attention.
In Montag’s world, firemen don’t put out fires. They start them. A fireman’s job is to burn books. Houses are fireproof, so it is only the books and insides that burn.
The fire serves two purposes. It gets rid of the books, and serves as a message and warning.
Always at night the alarm comes. Never by day! Is it because the fire is prettier by night? More spectacle, a better show? (Part I)
Besides being the show, the fires are warnings to others not to keep books in their houses. Alarms are set off by neighbors who suspect books, and naturally they would want to see the fire in all its glory, so they set the alarm off at night.
At this point in the book, Montag likes being a fireman and never questions anything. When the woman chooses to burn herself with her books, he gets curious. He wants to know why anyone would do that. He wants to know what is in the books, so he steals one to see what it's all about.