Actually, most correctly in the English language, there is no such thing as 12 am and 12 pm. There is only 12 noon and 12 midnight.

See: The National Maritime Museum, Greenwich, states:

To avoid confusion, the correct designation for twelve o'clock is 12 noon or 12 midnight. Alternatively, the twenty-four-hour-clock system may be used. The abbreviation a.m. stands for ante-meridiem (before the Sun has crossed the line) and p.m. for post-meridiem (after the Sun has crossed the line). At 12 noon the Sun is at its highest point in the sky and directly over the meridian. It is therefore neither "ante-" nor "post-".[8]

http://en.wikipedia.org/wiki/12-hour...n_and_midnight