翻译：They change from day to day.
day by day ：一天天 She is getting better day by day. 她的健康狀況一天天好起來。
day after day ：日復一日 He was doing the same thing day after day. 他天天做同樣的事。
I hate this ... They emphasize that you should use 正在 for present continuous tense (with an -ing), and then they make an exception for this sentence.