Rule: on is used with the days of the week or month (on Sunday, on February 15, etc.); at is used to denote the exact time (at dawn, at 10 o’clock, etc.); and in is used with a period of time (in spring, in the evening, etc.).
Incorrect Usage: (a) The delegates will arrive at Monday.
Correct Usage: (a) The delegates will arrive on Monday.
(c) My mother goes for a walk in the evening.