daily: is used with reference to the ordinary concerns of the day or daytime and may refer to weekdays as contrasted with holidays and weekends and may also imply an opposition to nightly: the ~ grind.       daily grind每日枯燥又费力的劳动，又如daily lives日常生活/daily concern每日的忧虑/daily wages日薪/daily necessities日常必需品/daily exercises每日锻炼/China daily中国日报/daily capacity日产量/daily occurrence日常发生/daily bread每日面包，比喻日常生计


diurnal: is used in contrast to nocturnal and occurs chiefly in poetic or technical contexts: ~ animals that are active by day.     如diurnal hours/flowers白天的时光/白天开的花


quotidian: emphasizes the quality of daily recurrence and may imply a commonplace, routine, or everyday quality to what it describes: found solace in ~ concerns.   如the car sped noisily off through the quotidian traffic汽车嗡嗡地飞速穿过日复一日的车流


circadian: a chiefly technical term, differs from daily or quotidian in implying only approximate equation with the twenty-four hour day: ~ rhythms in insect behavior.     如circadian clock生物钟/生理钟，a circadian rhythm生理节奏

daily每天: 指日常的周内的白天的，diurnal白昼: 用于诗歌或专业文章，和nocturnal夜晚相对，quotidian每天平常的: 强调日复一日重复寻常，circadian大约每天一次，生理节奏的: 专业术语，仅表示大约24小时


记忆方法: 1）首字母DDQC想成旦旦出去<==每日       "旦"本意是日出天亮、早晨，引申为白昼。 尚书名句"日月光华，旦复旦兮"是说日月的光辉，天天都照耀着大地，比喻自强不息。是复旦大学校名出处，西湖边有"光华复旦"牌匾

         2）每日的意思是每一天mean of each or every day.