grammar - When do you mark きょう or other similar relative time expressions with は

I've been trying to make proper sentences using the word きょう when I realized that I'm not really actually sure when I'm suppose to mark it with は. I know relative time expressions tend to stand alone meaning it's not followed by に and because of this its not uncommon for it to be followed directly by another word with no particle in between. This is where I start to get confused however when I see a sentence like this one

明日{あした}きます

and then a sentence like this

今日{きょう}は 京都{きょうと}に いきます

I'm pretty sure the reason I'm having a hard time understanding this is due to my terrible understanding of は, regardless any help on when I should mark a time like this with は and when it would be better left unmarked would be appreciated.

Answer

1) 今日は、京都に行きます。
2) 今日、京都に行きます。/ 京都には、今日行きます。

I would use #1, with a stress on 京都, as a reply to "Where will you go today?" or "What will you do today?", and #2, with a stress on 今日, as a reply to "When will you go to Kyoto?" Here, the は is the topic particle. 今日は in #1 and 京都には in #2 are old information (既知情報), and 京都に(行きます) in #1 and 今日 or 今日行きます in #2 are new information (新情報).

You would use #1 in a conversation like this:

(on the phone)
A: もしもし。今日、暇？今から遊びに来ない？

B: あ、ごめん、今日は京都に行くから無理。(not just 今日)

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The は can also be the contrastive particle. For example:

1') 昨日は大阪に行きました。(でも、)今日は京都に行きます。
I went to Osaka yesterday. (But) today, I will go to Kyoto.
1") 今日は、京都に行きます。 (with a stress on 今日は)
Today, I am going to Kyoto (but on another day I didn't / won't go there).

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When I just tell someone that I am going to Kyoto on that day (i.e. when 今日 and 京都 are both new information), I would use:

2') 今日、京都に行きます。(without a stress on 今日)

for example, in a context like this:

(on the phone)
母: もしもし。
娘: あ、お母はん？あのさ、今日、京都行くねんけど、なんか伊勢丹で[買]{こ}うてきてほしいもんとかある？ (not 今日は.)