Those Chinese Characters on the Door
It's a homophone, a play on words. The character that is hung upside down is
Fú - 福, pronounced “foo”. It means fortune or luck.
The pun - In Mandarin, saying Fú Dào Le means "luck or fortune has arrived". But the word "Dào" can also imply to fall down or turn upside-down. So, literally turning the character 福, Fú, upside-down is a play on words implying fortune has arrived.
On the door - You'll see the character, usually written in gold on a red background, hung on doors across the country by Chinese hoping for good fortune for the new year. The decorations are often left up all year so you may see it at any time. And why not? Everyone needs a little fortune heading their way.