adverb
day after day; for days on end
Emphasizes a continuous stretch of many days, often with a sense of monotony or endurance.
幾日も幾日も雨が降り続いた。
It rained day after day.
彼は幾日も幾日も同じ作業を繰り返した。
He repeated the same task for days on end.
毎日 simply means 'every day' without the emphatic, continuous nuance of 幾日も幾日も.
連日 also means 'day after day' but is more formal and often used in news or business contexts.
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.