Day and Night
There are 24 hours in a day. The day is divided into day(time) and night(-time). Daytime is from sunrise (this varies, but we can say approximately 6am) to sunset (we can say approximately 6pm). Night-time is from sunset to sunrise.
Every day starts precisely at midnight. AM (ante-meridiem = before noon) starts just after midnight. PM (post-meridiem=after noon) starts just after midday. This means that 12am and 12pm have no meaning.
This diagram shows the cycle of a 24-hour day and the words we use to describe its parts. The day starts at midnight (at the bottom of the diagram).
1. the 24 hours between one midnight and the next 2. the time between sunrise and sunset (as distinct from night)