Day-after-day is an idiom used to describe something that is happening repeatedly or consistently. There are several synonyms that can be used in place of this phrase such as daily, regularly, consistently, routinely, persistently, incessantly, unceasingly, relentlessly, tirelessly, continuously, unswervingly, and unfalteringly. Each of these words carries a slightly different connotation, but they all convey the idea of something happening repeatedly over time. These synonyms can be used in a variety of contexts, from describing a daily routine to a consistent pattern of behavior or thought. By using one of these alternatives to day-after-day, your writing can become more varied and descriptive.