It was 1988 when we first traveled to Europe = It was 1988 in which we first traveled to Europe =IIt was 1988 when we first traveled to Europe = It was 1988 in which we first traveled to Europe =It was 1988 which we first traveled to Europe in =It was 1988 we first traveled to Europe in .这里which为什么能省略的啊

用作时间状语的关系副词when通常不能省略，但有一种特殊情况，即用于day, year,
time等少数几个词后时可以省略(也可换成that)。如：
That was the year (that) I first went abroad.就是那一年我第一次出国了。
I'll never forget the day (that) we met.我永远也忘不了我们见面的那一天。
The second time (that) I saw him was in 2000.我第二次见到他是在2000年。
I don't know the exact time (when/ that) the sports meeting will take place.
我不知道运动会举办的确切时间。
你的句子可以理解为这语法规则，希望有助于你，建议你自己不要使用，我一直认为我们不是“大家”，只有老老实实用公认，无歧义的英语。欢迎探讨，祝你进步！

第一句中先行词时间1988在从句中做时间状语引导词不能省略，但是后面的几个句子里时间在从句中都充当了宾语，所以可以省略which

楼主的几个句子,意思一样,用不同的从句表达了相同的意思.
首先,第1个句子,It was 1988 when we first traveled to Europe ,是when引导的时间状语从句.it 指代的是“时间/ 日期”.这个句子就像“It was midnight when my father came home yesterday.”（“昨晚我爸爸回家时,已经是深夜.”）
第2个句子,It was 1988 in which we first traveled to Europe,是which引导的定语从句,先行词1988,关系代词which在从句里做宾语（in的宾语）.介词in前置了.介词前置,关系代词不能省略,所以这里的which是不能省的.
第3个句子,It was 1988 which we first traveled to Europe in,这个句子同第2个句子一样,只是介词in保留在原来的位置.关系代词在定语从句里做宾语（无论是做及物动词还是介词的宾语）时,可以省略的.所以这里的which可以省略,省略后就成了楼主的第4个句子.
总结：当定语从句的关系代词做定语从句里的宾语时（如果是做介词的宾语,而该介词没有前置时）,可以省略；如果介词前置了,则不能省略.
有趣的是,楼主的句子还可以改成第5个句子,而意思基本保持不变,这就是强调句：It was in 1988 that we first traveled to Europe.
供你参考,

只有当关系代词在宾语从句里做宾语时,才可以省略.
This is the man [who] you are lookng for. 这里的who可以省略, 因为who 在宾语从句里做宾语，不知道这么说对不对，希望采纳