知乎，让每一次点击都充满意义 —— 欢迎来到知乎，发现问题背后的世界。 "Saturday" is a defined period of a standard unit of your time (days), if it comes about on Saturday, it transpires only on Saturday. Whereas "at nine o'clock" implies setting up at 9, but continuing for an adaptable period of time; in the same