When we talk about something that is happening gradually, we often use the phrase "day by day." However, there are several other synonyms that you can use instead of this phrase to convey the same meaning. For example, "step by step" can be used to talk about a gradual process, while "gradually" is a simple word that means the same thing. Other synonyms include "little by little," "slowly but surely," and "inch by inch." Each of these phrases means that something is happening slowly or gradually, and can be used interchangeably with "day by day" depending on the context.