Difference between 上旬 and 初旬

Question

According to the dictionary, both 上旬 and 初旬 mean "the first ten days of the month approximately". I was wondering if there was some kind of nuance between the two or if there were strictly equivalent (or maybe it is simply usage, e.g. formal/informal). I saw things online like "初旬 is used mostly for the very beginning of the month" and "初旬 can be used less strictly and can work until the 12th for example" which seem a little contradictory. Thank you in advance if you could enlighten me.

Answer 1

A month consists on average of approximately 30 days. These are traditionally subdivided into the three 1/3 month phases 上旬{じょうじゅん}, 中旬{ちゅうじゅん}, and 下旬{げじゅん}: days 1~10, 11~20, 21~ respectively. These three are thus each of an equal fixed duration and (obviously) don't overlap each other.

The less commonly heard 初旬{しょじゅん} on the other hand, simply means "the beginning of the month", and as such isn't a subdivision of the month, nor is it part of some set like the ones above. Hence it doesn't have a clearly defined duration.

Counting the 12th as 初旬 feels like a bit of a stretch to me; but again, this being a relatively subjective term, I guess it could work. In particular for January when people often have (most of) the first week of the year off.

Answer 2

All the dictionaries I checked (including 広辞苑, 精選版日本国語大辞典, デジタル大辞泉 and 明鏡国語辞典) define 初旬 as something like 月の初めの10日間。上旬。 or simply 上旬。. Therefore, 初旬 refers rather strictly to the 1st to the 10th day of the month, making it completely synonymous with 上旬, at least according to these dictionaries.

That being said, as others have point out, while 上旬 is ubiquitously understood as the first third of the month, I do feel 初旬 is used less strictly. Practically, if you want to play it safe, I would recommend using 初旬 to refer to approximately the first 5 or 7 days of the month. Regardless of the situation, I do not think the 12th is part of 初旬, although it's certainly in 月の前半. 初旬 is not very common, so an even safer approach might be to avoid using 初旬 entirely, and use 月初【げっしょ】 or 月初め【つきはじめ】 instead when vaguely referring to "the beginning of the month".