陰陽, Yin and Yang means the shady side and the sunny side. It's the principal of Taoism.
Taoism is also deeply connected with the tea ceremony. In terms of number, Yin is even and Yang is odd. We prefer odd numbers because you can't split into two. Even if we receive four guests, we prepare five sweets on a tray. If we want to use the same designed or coloured utensils, we put three of them rather than two.
This thought is also taken in our daily life. When you buy some rice bowls, one set comes with five bowls. When you prepare some money for celebration, especially for wedding, the number of bills you put in a envelope must be odd number, e.g., 20 000 yen should be a 10 000 note and two 5 000 notes. This shows you hope they will not split.
When the odd number is doubled, even better. Some days of a year are traditionally cerebrated in Japan. 1st January, New Year's Day. 3rd March, Girls Day. 5th May, Boys Day. 7th July, Star Festival. 9th September, Choyo(double Yang). How about your birthday?
Welcome to the land where we still keep tradition from a 1 000 years ago!