Particle に and relative time expressions

Question

Many questions have been asked regarding the correct usage of the に particle to indicate the time when an action occurs and the matter appears to be more nuanced than it may seem, at least to beginners. As if things weren't complicated enough, textbooks tend to oversimplify or state half-truth, so here is my version of the question. As far as I've come to understand, there isn't really a rule prohibiting the usage of specific words with に, rather (in most cases) they would make much rarer nuances. One "rule" I've run into reading books and online is that

1. Relative-to-now time phrases should not be followed by に.

An example of this: 今日, 昨日, 今朝. These expression clearly convey a relative time with respect to the present. If I write a message on September 11, 今朝 will be the morning of that very day, 昨日 will be September 10 and so on.

Nonetheless, there are other words that I find difficult to understand how they work, for example:

• 朝 (or 夜): it's very clear (maybe excluding some specific nuance in literature) that 朝 does not require に. Nevertheless, in contrast to 今朝, it doesn't occur to me that 朝 indicates a time relative to the present: it could be the morning of any day in principle, or it could mean everyday in the morning;
• 週末: in this case, the particle に seems to be used (or at least it) and it makes sense according to the rule above, since this is not 今週末 (this weekend).

On the other hand, I've also found other versions of the "rule" for the usage of this particle:

2. に is used to indicate specific moment in time (e.g. what time you do something)

Or the even more controversial

3. に should not be used after some specific words (including also 朝 and 夜)

The last one may be an oversimplification for beginners, while I can't seem to understand how to connect 1 and 2. Probably the "specific" in 2 should be understood as absolute. For example, if I say 八時に, I'm considering an absolute point in time (because of course the day is implicit in the context), which is not defined in terms of the present (in contrast to 今朝, for example). Still, I can't really see what to do with 朝 and 週末 in either case. Could you please provide some clarity - possibly sticking to my examples and pointing out possible errors?

Answer 1

There is a blogpost that explains this question very well, but it's written entirely in Japanese. I would encourage you to read the original if it is within your Japanese level, but if it's a little too hard, here is a summary(not a direct translation) in English:

There are three categories of words when considering whether に is used

1. Those that must not use に

These are words that are, as you have already stated in your question, relative to NOW(今). Such words cannot be a specific point in time on its own, such as 今日、昨日、明日、今週、来週、今月、来月、先月、今年、来年、去年. There specific reference depends on the date and time they were spoken/written, and without context, one cannot point them out on a calendar.

2. Those that must use に

These are words that can be specified, or pointed out, on a calendar or a clock. These are ones such as 八時、2024年、七月、一月一日, etc. If I ask you to point out 八時 or 1999年 on the clock or calendar, you can do it. If I ask you to point out 明日 and without telling you the date of today, then you cannot do it.

3. Those can be with or without に

These are the ones that can be used either way. There are two types of words in this category:

One is a relative time not relative to NOW, but relative to another set time. These would include things like 卒業の翌日(the day after graduation). Even if 翌日 is relative, it's not relative to 今 but to graduation, whose time is considered a set time and yes you can point it out on a calendar.

The other is a period of time, aka those that refer to a period of certain length in time. Examples of this would be 春、夏、秋、冬, or 夏休み, etc. I believe 週末、朝、夜 also belong to this category, as they are time periods, thus can be used with or without に. Although 週末、朝、夜 are not mentioned directly in the article, the comments down below(and responses from the author) on the same post corroborate this usage(see screenshots below):