''This, the fifteenth day, is the time . . . when the white disk of the moon hangs in the void, and when the tinted clouds first begin to scatter, winecups are arranged and bowls washed amidst the noisy hubbub of the children. Verily it is what one may call a beautiful festival.'' CHINESE housewives will undoubtedly be the first to agree with this 1936 description of the Mid-Autumn Festival as this is one festival which doesn't keep them slaving over a hot wok. Food, of course, does figure prominently in the celebrations, but it's bakers and Mother Nature who prepare the spread. From bakers come mooncakes of sculptured pastry and golden moon-like egg yolks. From Mother Nature come fruits. Those most appropriate must match the moon in roundness, and lately in Hong Kong, three are popular: the pomelo, the crystal pear, and the persimmon. Crystal pears truly resemble the full moon. In perfect yin-yang counterpoint, they are as light and delicate in appearance, flavour and texture as mooncakes are dark, rich and dense. Measuring between seven to 10 cms in diameter, they have the crisp apple-like texture of Asian pears. (N.B. Apply suction if you bite into one lest the abundant juice run unceremoniously down your chin.) The green pomelos in the market are choice specimens, also round, with a relatively thin skin (as pomelos go) and a superbly sweet flesh (guaranteed to be less bitter and dry than local versions sometimes are). Select them by weight - the heavier the better. Peel them from the top down in one piece, leaving the sections like petals attached at the base. In earlier times these skins then became candle holders; with a candle inserted in the centre, they could be pulled along the ground on one end of a string by a child. As for persimmons, of the many varieties grown in China earlier in the century, only two or three now typically reach Hong Kong. They can all generally be distinguished from other fruits by the presence of a calyx of four leathery brown ''petals'' at thestem end. From there, characteristics diverge. The most common variety has a shiny, smooth orange skin, rather squat shape and soft texture; another has a rough skin coated in powder (harmlessly edible), a rather upright shape, and crisp texture. Peel them all before you eat them, and discard the seeds. But far and away the unsung treasure of the Mid-Autumn Festival markets - the one truly evanescent fruit which is available briefly now and at no other time of the year - an item as useless as it is bizarre - is the Water Caltrops (ling gok in Cantonese). These are black, shiny, hard and woody, in the shape of a brace of water buffalo horns but measuring only about five cms from tip to tip. Vendors of root vegetables and water crops (such as lotus) typically sell them because these, too, grow in ponds. The edible bit lies inside: a white, starchy nutmeat, not unlike a dry potato in texture and taste. They are an ancient crop, known since the Han Dynasty (200 BC), and still commonly sold (simply boiled) by street vendors in more northern China where people habitually crack them open with their teeth, to the chagrin (or glee) of local dentists. Relatives of the plant include the Singhara nut of India, Ceylon, Thailand and tropical Africa, and the Jesuit's nut, an aquarium plant. The shape of the nut is far more interesting than its taste, so perhaps the best use of the fruit is as a talisman. They dry nicely, simply exposed to dry air (or air-conditioning), and can then be threaded through the centre. Strung on silk threads with a bit of decorative macrame they make great (organic but sinister) dangles for the rear view mirrors. of taxi cabs.