一个英语网站上，提到一个关于星期+时间前介词的用法，对此有疑惑

When you combine a day of the week and a part of the day, you do need a preposition, except with noon and midnight. The time of day goes after the day.

横线处意思是否表示“除了noon和midnight，星期+一天当中某个时间前用介词”？如果按横线处我理解的没错，那么这个英语语法项又给出3个例句让我不理解：

I will see you Monday night.
Let's meets Sunday afternoon.
I will see you Friday at noon.

为何句1、句2中的Monday night和Sunday afternoon前没有介词？另外句3中的noon前面有介词？