section_path: Horology/Week
section_name: Week
paragraph_text: Week
  
  As early as the Bronze Age Xia dynasty, days were grouped into nine- or ten-day weeks known as xún (旬). Months consisted of three xún. The first 10 days were the early xún (上旬), the middle 10 the mid xún (中旬), and the last nine (or 10) days were the late xún (下旬). Japan adopted this pattern, with 10-day-weeks known as jun (旬). In Korea, they were known as sun (순, 旬).
  
  The structure of xún led to public holidays every five or ten days. Officials of the Han dynasty were legally required to rest every five days (twice a xún, or 5–6 times a month). The name of these breaks became huàn (澣; 浣; ' wash ').
  
  Grouping days into sets of ten is still used today in referring to specific natural events. "Three Fu" (三伏), a 29–30-day period which is the hottest of the year, reflects its three- xún length. After the winter solstice, nine sets of nine days were counted to calculate the end of winter.
  
  The seven-day week was adopted from the Hellenistic system by the 4th century CE, although its method of transmission into China is unclear. It was again transmitted to China in the 8th century by Manichaeans via Kangju (a Central Asian kingdom near Samarkand), and is the most-used system in modern China.