定语从句中出现on which时 ,是不是总是when的意思？比如？

不一定。这要看关系代词which所代替的词是哪一个了。如下面的几两种情况。
I will never forget the day on which I saw her for the first time.(on which=when)
we came to the playground on which some boys are playing football.(on which=where)只有这两种情况。

不是 也可能是在某东西上

当定语从句所修饰的先行词是表示时间的名词时，用when，在定语从句中充当时间状语。在口语中可以用that代替when，也可以用介词+which来代替when。 如：It was raining on the day when(on which) Peter arrived.
具体看先行词前是否可以搭配这个介词。 如 i will never forget the day when i met him. 这个句子的when 可以用 on which 代替,因为我们平时的搭配就是 on the day。 但是如果句子是　i will never forget the year when i met him. 这个句子的when 就不可以用on which 而要用　in which，　因为我们平时不说 on the year而说 in the year 。
有时也会用到at /during+which 如.That was the time when(= at which) he arrived,
或者 it rained heavily when (during which) we stayed together.

当然也有可能是where的意思 比如 this is the desk on which i found my pen这里的on which就等于when 所以要根据意思判断的,