Almost similar looking sentences with and without に particle

Question

I am having confusion with the follow two sentences. Currently I am preparing JLPT N5 so please answer accordingly (I can't understand very advanced explanations).

私は日曜日国へ帰ります

私は誕生日に国へ帰ります

So there are expressions of time where we should use に and some where we shouldn't . Can anyone classify for which expressions of time we should に particle and where we shouldn't. I read that for specific "time" we should use に but then also it seems like 日曜日 is a specific time. So why are we not using に particle here. These examples are from a book.

Answer 1

It's said that words that mean absolute time are accompanied with に while relative ones are not.

今日、きのう、明日、今週、先週、来週 or 今年 are ones that are used without に to indicate when something occurs. (Of course, they can be an (indirect) object in a sentence like (予定を)今日にする, besides that.)

日曜日 or 週末 are ones that might take に.

誕生日 or ◯月◯日 are ones that almost always call for に.

Answer 2

Check these sentences which I wrote in English below.

私は日曜日国へ帰ります → I will go to (my or someone's) country on (this, next or someday) Sunday.
私は誕生日に国へ帰ります → I will go to (my or someone's) country at (my or someone's) birthday.

As you can see my sentences, these Japanese sentences have some unclear points. In short, these are lack of "whose?" and "when?".

If I write Japanese sentence, for example, it is below:

I will go to my country on next Sunday. → 私は次の日曜日に自分の国へ帰ります。
I will go to my country at my mother's birthday. → 私は私の母の誕生日に自分の国へ帰ります。

Hope it helps.

Thanks

Answer 3

Both sentences are OK.

The difference is the 1st one is more colloquial.