25th day of the month; the 25th
Calendar-date use. It refers to the 25th day within a month and is read regularly as にじゅうごにち.
会議は２５日に延期された。
The meeting was postponed to the 25th.
締め切りは二十五日です。
The deadline is the 25th.
25 days
Duration use. It counts a span of twenty-five days, as in ２５日かかる or ２５日間; context distinguishes it from the calendar-date meaning.
この調査には２５日かかった。
This survey took 25 days.
彼は二十五日間
My salary is paid on the 25th of every month.
He practiced every day for 25 days.
その工事は２５日間かかった。
The construction work took 25 days.
Please submit the report by the 25th.
This procedure takes 25 days.
２５日 (にじゅうごにち) is read regularly, while 二十日 (はつか) uses an irregular traditional reading for the 20th day/20 days, so learners often mix up the reading pattern.
２５日 (にじゅうごにち) uses the plain にち reading, whereas 二十四日 (にじゅうよっか) for the 24th/24 days uses the irregular よっか pattern, so the two are easy to confuse when reading dates aloud.
Common numeric spelling for both a date, the 25th, and a duration of 25 days; full-width digits are used.
Kanji numeral spelling, used especially in more formal writing or when dates are written out.
Compressed kanji-numeral spelling; much less common than ２５日 or 二十五日.
Transparent compound of the number 二十五, 'twenty-five', plus 日, 'day'. This is a counting and date expression rather than a separate historical idiom.