Didn't you know the meeting was moved to the afternoon?

didn't 是 did not 的缩写，置于句首构成一般过去时的否定疑问句。这种形式表示说话人对某个事实感到意外或寻求确认，相当于中文的「难道……吗」。助动词 didn't 后接主语 you 和原形动词 know。