No black person was to ride a bus on Monday, December5, the day that Mrs. Parks came to trial.
请问，定语从句的先行词是day ,为什么用 that引导？ 一般不是用what 吗？
☆答：the day 在此与 Monday,December5同位，that 引导的是限制性定语从句。限制性定语从句以that 引导是常见的现象。句子里的日期已是特定的，其后的定语从句自然是限制性的了。例如:
Can you mention any one that we know?
Everything that could be done was done.
在表示时间的情况下，如果用 that可以代替at which， in which, on which则往往用that. 例如：
This is the time that the unjust man doth thrive.(莎士比亚)
By the time (that) you are ready, everybody will be gone.
The moment (that) the teacher entered ,everybody stopped their chatter.