When do i use は for きのう,きょう,あした?

Question

Example from Minna No Nihongo:

Lesson 4 : あした 6 時に起きます。

Lesson 9 : きょう は　早く　帰ります。

Why is は sometimes used with such a time words and sometimes not?

Answer 1

• As in many other situations 'は ha/wa' can denote a special significance this time. For example あした 6 時に起きます。plainly states that the subject of the sentence will wake up at 6 tomorrow.
• For the other example, きょう は　早く　帰ります。 there is a 'は ha/wa' inserted so it would add a nuance that I am waking up early today AND that this is not common for me to do this, i.e. normally I would wake up later.

In spoken English you might accentuate the "Today" as in "TODAY, I'm waking up early."

Answer 2

は:

1. Denotes the topic.

2. For comparing, emphasis

So in your second example

きょう は　早く　帰ります。

You wanna emphasize TODAY, you may always go back home late, but not today.