在定语从句中 when 的用法：
when 可以代替in/on which 引导时间定语从句.
例如 That was the year when he was born.
那是他出生的那一年.
Sunday is a day when people don't have to go to work.星期天是人们不用上班的一天
并不是说WHEN=介词+WHICH,而是说可以代替.
而且WHEN引导的句子中都直接和day,year,time,连用,修饰这些时间名词,today,yestoday,next month这些词的情况.