The Tong Shu (the "book that knows all," commonly the Chinese almanac) is the annual compendium of calendrical and divinatory knowledge used across the Chinese world to select auspicious days and hours for the actions of life and to avoid inauspicious ones. It is one of the most widely owned and consulted books in Chinese popular culture, historically found in a great many households, and it addresses the practical question of timing: on which day one should marry, hold a funeral, move house, open a business, sign a contract, begin building, travel, or undertake any significant act, and which days are unfavorable for each. It compiles an extraordinary density of traditional knowledge: the lunar and solar calendars and their correlation, the twenty-four solar terms that mark the agricultural year, the sexagenary cycle of the "heavenly stems and earthly branches," the auspicious and inauspicious associations of each day, astrological and feng shui data, guidance drawn from the I Ching and from systems of fate-calculation, along with charms, festival dates, and much folk lore.
The selection of auspicious dates it enables is a distinct traditional art (sometimes called ze ri, "choosing the day"). As a living practice, consulting the almanac, or a specialist who uses it, before weddings, funerals, and other important undertakings remains common across China and the Chinese diaspora, embodying the deep-rooted conviction that the timing of an action, in harmony with the cosmic calendar, bears on its success.