People describe a day in which everything goes wrong as a black day. 谁能解释下为什么该句中不用on which 而用in which (PS:我认为该句还原是in a day,关键是为什么不是on a day.有可能定从还原错了,见谅啊.)第二个,in a day 与 on a day 有什么区别?