定语从句中在哪些情况下用关系副词?

定语从句中在哪些情况下用关系副词?
*I will never forget the day when I first met you on the seashore.
*I will never forget the days which/that we spent together in Xueyao Middle School.
为什么上句用when而下句用which?
**This is the hospital where my mother works.
**This is the hospital which/that we visited the day before yesterday.
***The reason why he did not go to school is that he was ill.
***This is the reason which/that he gave his boss in the office .
还有这四句,为什么不用相同的词引导?
关系副词都可换成“介词+关系代词”的形式吗？