the international labor days which is also called may day.

你这句话没完.原句是：
The International Labor Days which is also called May Day,is on May 1st of every year.
主句是The International Labor Days（主语）is (系动词）on May 1st of every year(表语）.
which is also called May Day是which引导的定语从句,修饰主语,翻译成：
又被成为五一节的国际劳动节,是在每年的五月一日.
要想了解定语从句,一两句话说不清楚,请到百度文库去查,有很多的文章.

译文：国际劳动节，也称五一节

which在里面引导的是定语从句，which is also called May day是定语从句
定语从句是修饰名词或代词的句子，一般由关系代词或关系副词（如which、that、who、where等）引导，被修饰的这个名词或代词称为先行词
先行词在从句中是充当成分的，例句中的which指代的是前面的这个the internat...