所谓"三伏"，是指一年之中最热的时候。
      关于"三伏"的记载，早在秦代就出现了。根据《史记•秦纪》记载，那时就规定每年从夏至以后第三个庚日开始入伏，叫做"初伏"(或叫"头伏")，第四个庚日以后叫"中伏"，立秋以后第一个庚日开始为"末伏"。它包括初伏、中伏和末伏三个时期，一般出现在每年7月中旬到8月中旬之间。每年的"三伏"有长有短，长的时候有40天，短的时候只有30天。
      据《史记正义》释义："伏者，隐伏避盛暑也。"汉书《郊祀志》也云："伏者，谓阴气将起，迫于阳而未得升，故为藏伏。因名伏日。"所以"伏"就是"潜伏"的意思，既表示阴气受阳气所迫藏伏地下，又表示人们避开盛暑酷热。因此，从古代起，人们就有了很多三伏天避暑消夏的活动。春秋时期，三伏中的一天是要祭祀的。到了汉魏时期，三伏日就会举办宴会，人们喝酒、吃点心以消暑；汉和帝时期，曾命令在三伏日期间全天闭门休息，不做其他的事情； 到了唐朝，长安人则喜欢在伏日聚集于凉亭下，把西瓜、李子泡在凉水中以备食用，喝酒作诗，到晚上才回家。有的地方则搭起凉棚，招呼路上往来的客人，形成纳凉聊天的盛会。 

The term "Dog Days" (san fu) refers to the hottest period of year.

Records related to "Dog Days" date as far back as the Qin Dynasty. According to Historical Records • History of Qin, it was designated at that time that the third geng day (Geng is one of the ten Heavenly Stems used in the sexagenary cycle system) after the summer solstice marked the beginning of the "First Fu" (a fu is one of three ten-day periods of the hot season), the period after the fourth geng day was called the "Middle Fu", and the first geng day after liqiu or "Beginning of Autumn" would begin the "Last Fu". "Dog Days" include the first, middle, and last fu periods, generally spanning between mid-July and mid-August every year. "Dog Days" may last 40 days in some years, 30 days in other years.

The ancient work Proper Understanding of Historical Records explains, "During the fu one hides from the sweltering heat". The Treatise on Sacrifices chapter of the Book of Han also states, "Fu refers to the yin (cold) air repressed by the yang (warm) air, thus concealed by it. Hence the name fu." So fu means to hide or conceal, as the cold air is forced underground into concealment by the warm air, and people also conceal themselves from the torrid midsummer heat. Thus, from ancient times, people developed many activities to escape the heat of Dog Days. During the Spring and Autumn Period, one day during Dog Days was designated for sacrificing to the gods. During the Han and Wei dynasties, people held banquets, enjoying wine and desserts as a relief from the summer heat. Emperor He of Han Dynasty decreed that during Dog Days, everyone should close their doors and rest all day. By the Tang Dynasty, however, the people in Chang'an preferred to gather under pavilions, soaking watermelons and pears in cold water, drinking wine and writing poetry, returning home only after dark. In other places, people would erect tents, calling out to passers-by to join them for cool shade and merry conversation. 

• beginning [bi´giniŋ]   n.开始，开端；起源   (初中英语单词)
• conceal [kən´si:l]   vt.藏；隐瞒   (初中英语单词)
• relief [ri´li:f]   n.救济；援救；减轻   (初中英语单词)
• emperor [´empərə]   n.皇帝   (初中英语单词)
• everyone [´evriwʌn]   pron.=everybody 每人   (初中英语单词)
• writing [´raitiŋ]   n.书写；写作；书法   (初中英语单词)
• poetry [´pəuitri]   n.诗；诗意   (初中英语单词)
• related [ri´leitid]   a.叙述的；有联系的   (高中英语单词)
• historical [his´tɔrikəl]   a.历史(上)的   (高中英语单词)
• heavenly [´hevənli]   a.天的，天空的   (高中英语单词)
• underground [,ʌndə´graund]   ad.&a.地下(的)   (高中英语单词)
• dynasty [´dinəsti]   n.王朝；朝代   (英语四级单词)
• treatise [´tri:tiz, -tis]   n.(专题)论文   (英语四级单词)
• midsummer [´mid,sʌmə]   n.仲夏；夏至   (英语四级单词)
• concealment [kən´si:lmənt]   n.隐藏，隐瞒   (英语六级单词)
• calling [´kɔ:liŋ]   n.点名；职业；欲望   (英语六级单词)