什么情况下在表示时间的状语前不加介词？

在平时的学习中,我们注意到一般的时间状语前都有介词in/on/at或for 等,而实际上在英语中,有不少表示时间的状语前是不加介词的.
1.在 today, tomorrow, yesterday, the day before yesterday, 等表示时间概念的名词前一般不用介词. 例如:

1) What are you going to do tomorrow?

你们明天打算干什么?

2) We have had six lessons today.

我们今天上了六节课.

3) We had a good time the day before yesterday.

我们前天玩的很开心.

2.由this, that ,these, those 等用于表示时间概念的名词前,

前面往往不加介词.例如:

1) That morning he got up very early.

那天早上他起得很早.

2) What are you busy with these days?

这几天你在忙些什么?

3) We will have a big gathering this coming Saturday.

下星期六我们将进行一次大聚会.

3.由next, last, the next, the last 等用于表示时间概念的名词前时,前面不用介词修饰.例如：

1) We planted many trees last spring.

去年春天我们栽了许多树.

2) She said she would pay a trip to Suzhou the next month.

他说他下个月将去苏州.

3) The very next time I saw her, she was