In Japan, certain numbers are lucky and unlucky. It's really important to know these numbers because if you don't, you could be accidentally telling someone you'd like them to suffer a slow agonizing death when you're giving them omiyage (お土産, souvenirs) from your trip to Kyoto.
Death, agony and suffering
First, let's get the bad ones out of the way. Four is an unlucky number in Japan because it sounds like shi (死 – death). This is why there are two readings for the number four, shi and yon. Whenever possible, people try to avoid using the deathy one.
The same is true of the ku (九 – nine), which sounds like ku (苦 – suffering, agony or torture). Similarly to four, there are two readings for nine – ku and kyu.
Some buildings such as hospitals don't have fourth or ninth floors, although I've never personally encountered one. Maternity wards may not have a Room 43 because it sounds like shisan (死産 – stillbirth).
Certain license plate numbers are not used such as 42, which sounds like shini (死に – to death); 49, which sounds like shiku (敷く – to run over); 42-19, which sounds like shini iku (死に行く – to go and die); 42-56, which sounds like shini-goro (死に頃 – time to die); and 24, which can be nishi (二死 – two deaths or two out if you're a baseball fan).
Some of the Yakuza's scarier members use their license plates to express their contempt for their own mortality by choosing 4444. That's quite a bit of death and a car with this plate is one you don't want to cut off on the highway.
You have to get creative when you have a number that sounds like death.
Like many countries throughout the world, Japan considers the number seven lucky. This is not imported, but steeped in the country's religious traditions. Seven is an important number in Buddhism. Japanese Buddhists celebrate a baby's seventh day and mourn the seventh day after a person dies when the soul is said to cross over.
In Japanese folklore there are the Shichifukuin (七福神 – the Seven Gods of Luck). Tanabata (七夕 – Evening of the Seventh) is an important summertime holiday that's celebrated on July 7th (7/7). The number seven also makes many appearances in pachinko parlors and scratch tickets.
Although slightly less well-known, eight is also a lucky number. This is due to its shape – 八. Called suehirogari (末広がり), it's lucky because it widens at the bottom which reminds one of prosperity and growth.
Japanese people tend to be quite superstitious and this is why lucky and unlucky numbers are important. You should never give someone four or nine of something. Gifts are given in threes and fives instead.
On a related note, if you're looking for a nice quick read this weekend, check out The Thing About Luck, a story about the 12-year-old Summer whose parents get called away to care for relatives in Japan.