The 5th month of the Chinese lunar calendar is traditionally believed to be the "Evil Month". It is a time of hot, sultry weather which encourages the growth of harmful insects and germs which can spread infectious diseases of epidemic proportions. Dispelling unwanted disease and driving out evil are the main purposes of the Double 5th Festival. Ancient Chinese believed that realgar was an antidote for all poisons, and therefore most effective to exorcise evil spirits.
This month is also called the "Five-Yellow Month". The "Five Yellows" refer to yellow realgar spirit, yellow croaker, yellow cucumber, yellow eel, and the yellow yolk of the salted duck egg. So, on the Double-5th Day "Duanwu Jie", the 5th day of the 5th lunar month, local people, regardless of age, will drink some spirits mixed with realgar and children will have the Chinese character of "KING (王)" written on their forehead with realgar wine. As if to ensure complete protection, mugwort and realgar branches (leaves) are displayed at the gate or door of each house to drive away evil spirits. Lime is also spread at the corner of the house, the corridor and the sewer. During the festival time, women and children wear a "fragrant pouch" containing cattail. For thousands of years, this wonderful festival has been marked by eating zongzi (a pyriamid-shaped dumpling made of glutinous rice wrapped in bamboo leaves) and racing dragon boats. That is why the Double 5th Festival is otherwise known as the Dragon Boat Festival one of the most popular festivals in China.
Source: New Greater Hangzhou: A New Guide by George Chen