noun
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.
noun
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、毎日練習した。
He practiced every day for 25 days.
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.