（1）可引导限定性和非限定性定语从句，其先行词必须是表示时间的名词，如day，year，time等。when可用on which 来替换。
a.I'11 never forget the day when I met you.
b.I'11 never forget the day on which I met you.
c.I'11 never forget the day that I met you.（特别关注：此句中that用作关系副词）
d.April Fool's Day is that special day of the year when you should play a joke on someone! Children's favorites are to put salt in the sugar bowl for Dad's morning coffee or put chalk on a desk chair at school so the teacher gets a white backside! But remember, if you play a joke after 12 at noon, YOU are the April Fool!
e.We will put off the outing until next week , when we won't be so busy.
f.March 10, 1876 was the day when (= on which) the first complete sentence was sent over a telephone.
g.My favorite season is spring , when the earth seems born again.
h.The day may soon come when we don't bother to go to office but just work at home.
i.This kind of solar cooker can be used only in the daytime ______the sun is shining.
A. when
B. as
C. while
D. since
正确答案：A。
（2）紧跟在时间名词后边的关系副词，也可省去。
a.I'11 never forget the day I met you.
b.Most education experts today stress the importance of "life-long learning”. They point out that education never ends. It is a continuous process. The day we stop learning is the day when we die.
（3）注意：表示时间的先行名词，若定语从句不是缺状语，而是缺少主语或宾语，要用which或that来引导定语从句。因为用when来引导定语从句的前提条件是when在其引导的定语从句中作时间状语。
a.I'11 never forget the time which I spent on campus.
which作spent的宾语。
b.I'11 never forget the day when we first met in the park.
when作时间状语，修饰met。
c.The hours_________the children spend in their one-way relationship with television people undoubtedly affect their relationships with real-life people.
A. when
B. on which
C. that
D. in which
正确答案：C。
分析：这句话的定语从句中的谓语spend缺宾语，因此填入的关系词要充当这一宾语，所以选C。