'Midnight' refers to 12 o'clock (or 0:00) during the night. When using a 12-hour clock, 12 pm typically refers to noon and 12 am means midnight. While most people follow this convention, technically this isn't quite right. To avoid any confusion, 12 o'clock should be written as 12 noon or 12 midnight instead.

Is 12am in the morning?

12 a.m. is midnight. 12 p.m. is midday. After 12 a.m. it is morning. After 12 p.m. it is afternoon.

Is 12am the next day?

Each suceeding day therefore ends at midnight 12:00:00. The next day begins a nanosecond after midnight. So, it seems most accurate to say that each day ends at midnight, and the next day starts “immediately after midnight.”