It’s Friday 13th – need I say more?
In Japan, the number 13 is not considered particularly unlucky – that’s a superstition imported from the West. It’s not as if the Japanese are completely free of odd beliefs when it comes to numbers though. The numbers 4 and 9 are considered unlucky. 4 because its Chinese reading is “shi”, and the word for death is also pronounced “shi”. 9 is unlucky because its pronunciation as “ku” sounds like “suffering”. Like in the West, where the number 13 is often avoided in hotels (from floor 12 you go straight to 14) or airlines (no rows 13), the same holds in Japan for the numbers 4 and 9, although it seems to be more common not to have a 4th floor than not to have a 9th. Apparently, planes of All Nippon Airways have no seats with numbers 4 or 9, and many hospitals do not have rooms with these numbers. These beliefs spill over to other areas as well. For example, when giving gifts, you should always take care to give odd numbers – 3 or 5 plates for instance, not 4. For occasions where money is considered appropriate, like weddings, an odd amount (other than 9000) is better than an even one. The best would be a gift of size or amount including 8 though, as 8 is considered a lucky number. The kanji for 8 consists of two strokes that are farther apart on the bottom than on the top, which signifies that a better future lies ahead.
What I find very interesting is that there are lots of odd numbers in many normal packages: Meiji chocolate has 15 little pieces, there are 11 chocolate covered cookies and 5 chocolate buns per pack, and my favourite sweets – chocolate covered macadamia nuts – come in packages of 9 (the suffering is probably in the weight gain). And the last time we opened two 98g packs of chocolate covered almonds – one of them contained 23, and the other one 25. 
Coincidence? I’m not sure. But I might be eating too much chocolate…