There are many synonyms that can be used in place of the phrase "day after day." Some of these might include "repeatedly," "incessantly," or "consistently." Other options might include "endlessly," "unstoppably," or "unrelentingly." No matter which words you choose to use, the goal should be to convey a sense of continued repetition or ongoing activity. Whether describing a monotonous routine or an endless task, these synonyms can help emphasize the unyielding nature of the process, and convey a sense of inevitability or relentlessness. With a little creativity and thoughtfulness, it's easy to find alternative ways to express the idea of something happening "day after day".