On each successive day; gradually and steadily:day by day I grew worse...
The pressure for a full public inquiry is now growing day by day.
It had slowly dwindled day by day as the news of his accident became old hat.
When I was born I was very small, but I grew day by day and became a plump, chubby child.
Synonyms
gradually, bit by bit, by degrees, by stages, inchmeal, inch by inch, little by little, step by step, slowly, slowly but surely, steadily, progressively