First, about setting off firecrackers:
According to legend, in ancient times, there was a fierce monster called Nian, which was in the deep sea all the year round, but whenever the old and new years alternated, it would come out to destroy crops and kill people. Once it ran to the village to do evil again, and happened to meet two shepherd boys playing with bullwhip. I suddenly heard the sound of a whip in mid-air and ran away in fear. He ran to another village and saw a red dress hanging at the door of a family. He didn't know what it was, so he turned and ran away. Later, he came to another family, looked into the door, saw the lights inside, and felt dizzy, so he had to slip away again with his tail between his legs. From this, people realize that Nian is afraid of noise, red and light, so they think of many ways to resist it. For example, at the end of the year, people put up red couplets at home, set off firecrackers, burn firewood in the yard, and chop vegetables and meat with kitchen knives to make a sound, which gradually evolved into the custom of celebrating the New Year today.
Second, about posting couplets:
It is said that the custom of pasting Spring Festival couplets began in the post-Shu period about 1000 years ago. According to records, the original form of Spring Festival couplets is what people call "Fu Tao". There is a ghost world in myth, in which there is a big mountain, a big peach tree covering three thousand miles, and a golden rooster on the treetop. Whenever the golden rooster crows in the morning, the ghost who travels at night must return to the ghost domain. Shen Tu and Lei Yu stood at the gate of the ghost domain.
If the ghost does something unnatural at night, Shen Tu and Lei Yu will immediately find it, catch it, tie it up with a rope made of Miscanthus and give it to the tiger. So all ghosts are afraid of two people. So people carved them into peach trees and put them at their doorsteps to ward off evil spirits and prevent harm. Later, people simply carved the names of Shen Tu and Lei Yu on the mahogany board, thinking that this could also eliminate disasters. This kind of red board was later called "Fu Tao".