Why should the word "_" be posted at the wedding celebration? What is the reason?

In China, when people get married or hold any celebration, they always put red characters on the doors and windows. It is said that this custom is related to the story that Wang Anshi, the prime minister of the Northern Song Dynasty, had a double happiness when he was young. Twenty-three-year-old Wang Anshi went to Beijing to catch the exam one year and passed by Majia Town. It was getting late, so he decided to stay in town and have a rest. Walking in the street after dinner, I saw lanterns hanging outside the house of a large family, and the lights were flashing, which was particularly dazzling. When Wang Anshi came closer and looked closely, he saw a pair of half-pictures clearly written on the lamp, "Run the lantern, leave when the horse lights up, and stop when the horse lights up". Obviously, he was waiting for someone to make the bottom line. Wang Anshi couldn't help clapping his hands and praising: "Good right, good right!" The housekeeper who stood by immediately went in and told the owner of the house, Mr. Ma, but when he came out, Wang Anshi had disappeared.

The next day, Wang Anshi waved his hand in the examination room and answered freely. Seeing that he was clever and alert, the examiner sent someone for an interview. The examiner pointed to the flying tiger flag day in front of the hall: "Flying tiger flag, flag flying, flag rolling, tiger hiding." Wang Anshi immediately thought of half a pair of lanterns outside the stable. He replied without thinking: "The lanterns run, the lights go, and the lights stop." Examiners nodded frequently.

After the exam, Wang Anshi returned to Majia Town. Thinking of the help of lanterns, he went to Mr. Ma's house to see lanterns. The long-awaited housekeeper immediately recognized that he was the "xianggong" who praised Yu Lian a few days ago, and insisted on inviting him into the house. After sitting down after watching tea, impatient Commissioner Ma asked Wang Anshi to match a pair of lanterns. Wang Anshi immediately presented the examiner's first couplet: "Flying Tiger Flag, Flag Flying Tiger, Flag Rolling Tiger Hiding." Seeing that he was clever and neat, Mr. W immediately betrothed his daughter to him and proposed to marry him at Mafu on another day. It turns out that the pair on the lantern was made by Miss Ma at her husband's choice. ?

On the wedding day, the stable was beaming. Just as the bride and groom were worshipping heaven and earth, a newspaper came to report: "Wang daren is the top scholar, please go to Qionglin banquet tomorrow!" " "I was overjoyed. Mr. Ma was overjoyed and immediately reopened the banquet. In the face of double happiness, Wang Anshi wrote a big word "Xi" on red paper, which made people stick it on the door and recite it casually: "The couplet is a double happiness song, and lanterns are flying. "Since then, double happiness has spread among the people.