18 synonyms for day by day such as by degrees, a day at a time, and bit by bit. Day by day is a noun, and its antonyms are all at once and sporadically.
A day at a time, bit by bit, by degrees, by stages, daily, day after day, diurnally, every day, gradually, inch by inch, inchmeal, little by little, progressively, slowly, slowly but surely, steadily, and step by step.