If you planned to get a haircut today, think again. Apparently, it is a bad day for it. If you are meeting some friends the day after tomorrow, you are likely to have great fun. How do we know? Because the Chinese almanac says so. Also known as Tung Sing, the Chinese almanac is a calendar that predicts good or bad days for certain activities. It is also used to mark the seasons. For example, it is quite likely to rain tomorrow, and we will probably hear the first thunder of the year on March 6. But how are these special dates chosen? To understand it, you must first forget that a Chinese calendar is a lunar calendar. It is, in fact, compiled according to the movements of the sun, the moon, the planets and other heavenly objects, said Choi Hing-wah, an author of Gen Po Tong Chinese Almanac And Calendar. The art of compiling the Tung Sing began when the Chinese started to study the sky in ancient times. The earliest version of it was the record of cycles of day and night, full and new moon and the four seasons. 'The marking of the seasons was vital in ancient China because farmers relied on the almanac. It was compiled by the officials in court. Releasing the new almanac each year was a very important ritual,' Choi said. By the time of Shang Dynasty (1562-1066 BC), new elements began to emerge. 'Shang people practised rituals to look into the future,' Choi said. 'When results of these rituals were gathered, a pattern had emerged - certain days or hours had often been bad for some important activities in a specific location. 'Several books have been compiled since then, which we still refer to in carrying out calculations. So when it comes to the good or bad days, it is more a matter of statistics and algorithm.' By the Zhou Dynasty (1066-256 BC), the almanac was well-developed. It was made widely available in the form of paper-made books in the Tang Dynasty (618-907 AD) thanks to the invention of the printing press. 'Because it contains so much information, it is called Tung Shu. It means 'the book that contains everything',' she said. It later became Tung Sing because Cantonese-speaking people wanted to get rid of 'shu', which sounds the same as 'to lose' (while 'sing' sounds like 'to win'). Nowadays Tung Sing is more often referred to when choosing dates for important events, such as weddings and moving house. The book itself has also undergone changes. It now contains rhymes and thoughts of ancient thinkers, and ways of predicting one's own future. But Choi does not encourage people to follow the book slavishly. 'Tung Sing is meant to help people make changes to avoid bad luck,' she says. 'You still have to make plans according to your capacity and the slot of time available. You do not want to move to a new house when you are busy dealing with loads of projects, just because it is supposed to be a good day for moving. 'So when you have to do something at a certain time, and everything has been well prepared, that is when Tung Sing and geomancy can come into assistance,' she said.