CGEL英文版：P.688 Prepositions and prepositional phrases

On Monday morning, on Saturday afternoon, on the following evening, etc

illustrate an exceptional use of on with a complement referring to a PART of a

day, rather than a WHOLE day. This use also extends to other cases where the

time segment is a part of a day which is actually mentioned: on the morning

of 1 June, etc. But with phrases like early morning and late afternoon it is

normal to use in: in the late afternoon.

这里夸克的话里，划线的部分不知何意，也许是指与On Sunday， (On) that day之类相比较吧。否则，这不是废话么？管它On什么morning，难道还能指一整天？

关于的on/in, 夸克的意思是，

on the morning of 1 June （OK）

in the late afternoon. （normal）

但这里夸克没有进一步讲下去。

标红线的部分是关键点：which is actually mentioned. 换句话说，仅仅说 on the morning （of l June），没有这个打括号的部分，仅用on the morning，意思就很可能没讲完整。

1. He got up in the early morning.

2. He got up on the early morning.

如果没头没脑的突然来这么一句，1意思完整没问题，2可能会被听成没讲完，人家还会期待你讲下去。就是对句子1，人们对后面的of期待值不会太大，而对2，人们期待有个of之类的结构。 但即使加上of 结构，on the early morning of ... 很多人还是会认为不正确。