When is midnight 12 am or 12 pm?

It is NEITHER 12 am NOR 12 pm.

The correct time is ' 12 noon ' or ' 12 midnight '.

'am/pm' are the initials of the Latin phrase ' ante meridian/ post meridian ' . which means 'before noon/ after noon '.

So if it is '12 noon' how can it be before noon(am) or after noon (pm) . It is exactly '12 noon'.

A similar argument holds for '12 midnight' How can it be 'before/after noon , when it is exactly half way between the two previous noon and the following noon. Also at 12 midnight , there is the question of the day/date, which one do you select.

NB Yes people do say '12 am/pm' , but itis quite erroneous, and leads to confusion.

, that's why you are asking the question, because you are confused over it.