Translation guide
Expressing 'days' in Japanese depends on context: counting days, talking about days in general, or referring to specific days. The most common way is using the counter 日 (にち/か) or the noun 日 (ひ).
To say 'X days' as a quantity or length of time.
Use the counter 日 (にち) with 間 (かん) for a duration of days. Note irregular readings for some numbers.
三日間旅行に行きます。
I'm going on a trip for three days.
一週間は七日間です。
One week is seven days.
Without 間, 日 can also mean 'X days' in some contexts, but 日間 is clearer for duration. The reading changes for some numbers.
Using just 日 without 間 can be ambiguous; it might mean 'on the Xth day' instead of 'for X days'. Use 間 for clarity when expressing duration.
三日待ってください。
Please wait three days.
Referring to 'days' as a concept, like 'those days' or 'in the old days'.
Means 'days' in a general, often poetic or reflective sense, like 'everyday life' or 'the passing days'.
日々の生活を楽しむ。
Enjoy the days of everyday life.
The basic word for 'day'. Can be used in phrases like 'those days' (あの日々) or 'every day' (毎日).
Using 日 alone can be vague; often combined with other words for clarity.
あの日は楽しかった。
Those days were fun.
Means 'era' or 'period', often used when 'days' refers to a historical period.
Referring to particular days, like 'on those days' or 'what days?'.
Means 'day of the week'. Use when asking or stating which day of the week.
何曜日がいいですか？
What days are good?
Can mean 'day' as in a date or a specific day. Often used with modifiers.
休みの日は何をしますか？
What do you do on your days off?
日 alone can mean 'day' or 'days', but 日間 explicitly marks duration. Use 日間 when you want to avoid ambiguity, especially with numbers.
The counter for days has many irregular readings. Be careful with 1–10, 14, 20, 24, etc. Common ones: 一日 (ついたち/いちにち), 二日 (ふつか), 三日 (みっか), 四日 (よっか), 五日 (いつか), 六日 (むいか), 七日 (なのか), 八日 (ようか), 九日 (ここのか), 十日 (とおか), 二十日 (はつか).
My school days were fun.