Almost similar looking sentences with and without に particle


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.




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.

