春节贴“福”字，为什么要贴倒？

The third interpretation attributes the practice to Fu Jin, the Princes of Gong of the Qing Dynasty. Once, on the lunar New Year's Eve, the butler of the mansion of the Prince of Gong wanted to curry favor with his master.
第三种解释跟清朝的恭亲王福晋有关。某一年春节，亲王家的管家想讨主子欢心。

He followed past practice and had several large “fu” written and pasted on the front gates of the warehouse and the mansion. One of the men sent to do the pasting was illiterate and put the character upside down on the front gate of the mansion. Enraged, Fu Jin wanted to punish the perpetrator by whipping him.
他遵照传统，命人在库房和王府大门上贴了几个大大的“福”字。有个奴才因为不识字将王府大门的“福”字贴倒了。福晋因此十分生气，打算鞭打那个犯错的人。

The butler, who had the gift of the gab, hastened to go down on his knees and pleaded: “Your humble servant often heard people say that Your Excellency is a man of longevity and great fortune. Indeed, great fortune did arrive today; it is a good sign.” Fu Jin was convinced. “This is why the passers-by were saying that great fortune had arrived in the mansion of the Princess of Gong,” she thought, “Once an auspicious saying is repeated for a thousand times, my wealth could increase by 10,000 taels of gold and silver.”
这个大管家能言善辩，赶紧给主子跪下求情：“奴才常听人说亲王寿高福大造化大，如今大福真的到（倒）了，乃吉庆之兆。”福晋听罢心想，怪不得过往行人都说恭亲王府福到（倒）了，吉语说千遍，金银增万贯。

She then awarded the butler and the servant who pasted the paper upside down fifty taels of silver. Since then the practice of pasting “fu” upside down during Spring Festival has become a tradition followed by both imperial aristocrats and commoners.
福晋一高兴， 便重赏了管家和那个贴倒福的奴才。从此以后，春节倒贴“福”字就成了封建贵族和普通百姓的共同传统。