expression
day by day; daily; as days pass
Describes a gradual change or progression that happens over successive days. Often used with verbs of change like 増える, 変わる, 回復する.
日を追って彼の体調は良くなっている。
His condition is improving day by day.
日を追って感染者数が増えている。
The number of infected people is increasing daily.
A fixed expression composed of 日 (day), を (object marker), and 追って (te-form of 追う, 'to follow'), literally 'following the days'.