Very late at night, some time between midnight and dawn. Either you have to stay up very late, or you have to get up too damned early. Pronounced "oh dark thirty" or "zero dark thirty". (Derived from military time.)
My flight out's at 8 AM, so I'll have to get up at 0-dark-30 to get through security.
#dark 30#oh dark 30#ass-crack of dawn#zero dark 30#dark-thirty