Green’s Dictionary of Slang

(US) the point twelve in craps.

1919		C.E. Piesbergen Overseas with an Aero Squadron 56: ‘Whatta ’y’ shoot?’ [...] ‘Midnight!’.
1927		C. Sandburg Good Morning, America 14: Fate’s crapshooters fading each other, big Dick or snake/eyes, midnights, and deuces chicken one day feathers the next.
1949		(con. 1943–5) A. Murphy To Hell and Back (1950) 33: Snuffy rolls a twelve. ‘Hot damn. Midnight. A mighty hard point but I think I can make it.’.