day after day; for days on end
Emphasizes a continuous stretch of many days, often with a sense of monotony or endurance.
It rained day after day.
He repeated the same task for days on end.
Standard kanji spelling for this adverbial phrase.
Mixed kanji-kana spelling; less common but still encountered.
Reduplication of 幾日 (いくにち, 'how many days') with the particle も, literally 'many days and many days', emphasizing a long continuous period.