adverb
day by day; every day; by the day
Describes a gradual change or a recurring daily occurrence. Often used with verbs indicating increase, decrease, or progress.
日ごとに暖かくなってきた。
It's getting warmer day by day.
彼の日本語は日ごとに上達している。
His Japanese is improving every day.
毎日 simply means 'every day' as a routine, while 日ごとに emphasizes a gradual change or a daily progression.
日増しに also means 'day by day' but is slightly more literary and often used with intensifying changes.
Compound of 日 (hi, 'day') + ごと (goto, 'every') + に (ni, adverbial particle). The ごと element indicates 'each' or 'every', forming an adverb meaning 'with each passing day'.