How do you understand the Qinglong Jianxi posted when you get married?

The saying of "Green Dragon Seeing Joy": When people get married, a pair of "Blue Dragon Seeing Joy" is always posted diagonally on both sides of the door.

The origin of Qinglongjianxi. It is said that Emperor Qianlong came to the foot of Qianshan Mountain in Xi Zhong's hometown for a private visit incognito. On the way, he encountered a procession of people playing music and playing to welcome his bride. Emperor Qianlong pointed out that it was a day of five ghosts and it was not suitable to get married.

I wanted to find out more, so I followed the wedding procession to the door of the groom's house, stood by the door and leaned over to look in. When the bride got off the sedan and was about to pay homage to the groom, Emperor Qianlong shouted loudly: Who chose the day for the wedding? There was total silence.

After a while, an old Taoist came to the door and bowed politely to Qianlong. He said that it is not suitable for the five ghosts to marry today. Although it is an unlucky time, with the green dragon guarding the door, it will naturally turn the evil into good luck. Xiang, old man, please use the wedding bar. From then on, when getting married, a green dragon was affixed to show good luck.