One of the decorations is calligraphy. The most common word is fú (福), meaning happiness or fortune. But you’ll rarely see it upright.
In the Ming dynasty, the Emperor ordered every household to decorate by pasting fu onto their doors. On New Year’s Day, he sent soldiers to check. They found that one illiterate family pasted the word upside down.
The Emperor ordered the family to be punished by death. Thankfully, the Empress was there and came up with an explanation: “Upside down” (倒 / dào) is a homophone of “here” (到 / dào). When it’s upside down, it means that fu is here.
The explanation made sense to the Emperor, and he set the family free. From then on, people would hang the word upside down, both for fortune and in remembrance of the kind Empress.