"Flowers of plants and trees are in general five-pointed. However, flowers of snow, which are called ying, are always six-pointed."
"The reason why snowflakes are six-pointed is because they are only half-frozen rain (xian) (i.e. water) split open by violent winds, and so they must be six-pointed. If one throws a lump of mud on the ground it will splash into a radiating, angular petal-like form. Now 6 is a yin number; and gypsum also is six-pointed with sharp prismatic angular edges. Everything is due to the number inherent in nature. "
"Snow is the ultimate (state) of yin and completely possesses the number of Water (i.e. 6). Every snow-flake is six-pointed. Frost and snow are due to the condensation of rain and dew. Water is is generatd by Metal. A surplus of qi reveals the Mother (i.e. Metal). Hence frost and snow are all white."
Earliest Mentions Source: Li, Qi and Shu: An Introduction to Science and Civilization in China (available in Google Books online)
View a screen shot of the Woodcut by Olaus Magnus taken from an article by F. Eijgenraam NRC Handelsblad (14 Feb. 1991) (on page 8 of a very large pdf file)