Hi Kira,

Often in many languages, one cannot translate literally. Same is the case in Japanese.

Suppose you want to say your sentence on a Tuesday.

Normally we would say: 「土曜日はさむくなるよ・なります・なるでしょう・なるそうです。」

You can add 「来る土曜日は」「今度の土曜日は」、but this would rather be the case on

weather forecasts etc. and not in a normal conversation unless you want to speak about

next Saturday. l hope this was helpful.

This Saturday will be cold.

"This Saturday will be cold." How can you write it in Japanese?

Here is my sentence: この木曜日、天気は寒い。
It was corrected actually, but since I did not write its English version, I think that native speaker misunderstood me. "Saturday" in the phrase "This Saturday" hasn't come yet. By the time I said it, it could be Monday or Tuesday.
今日は木曜日、天気が悪くて寒い。今日は木曜日、寒い。

This Saturday will be cold.

I think the correct way of saying this is: 「この土曜日は寒くなります　OR   この土曜日は寒くなるでしょう」I don't think you really need the word 天気 in this case, but when you do, it will be like this: 「この土曜日は天気が寒くなります。」

I hope my answer was helpful!