It was 1988 when we first traveled to Europe = It was 1988 in which we first traveled to Europe =I

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,