Took FOREVER to find anything that remotely addressed the origins of "the crack of dawn"
(AT) THE CRACK OF DAWN: Early in the morning. The origin of this expression is uncertain. One writer suggests that �crack� is derived from the ancient meaning of sudden loud noise (since the word comes from Old English � cracian�,� to resound), because the sun comes up suddenly. Rudyard Kipling used similar imagery in his poem �Mandalay,� where �the dawn comes up like thunder outer China �crost the Bay.� On the other hand, �crack� may refer to a small space or opening�that is, the wedge of light that appears as the sun rises over the horizon. Whichever, the phrase originated in America in the late 19th century. It may already have been a clich� when W. Somerset Maughm wrote (�Catalina,� 1948, �He had slipped away at the crack of dawn.� (Facts on File Dictionary of Clich�s)
CRACK OF DAY (Originally) / CRACK OF DAWN: [late 1800s] Very early morning, daybreak. For example, �I got up at the crack of dawn.� The �crack� in this term alludes either to the suddenness of sunrise or to the small wedge of light appearing as the sun rises over the horizon. Originally the term was usually put as �crack of day. <1887: �At �crack of day,� as the sergeant of the guard expressed it, the stir of the camp was started by waking up the cook.���Outing�10.7/1> (American Heritage Dictionary of Idioms + Dictionary of American Regional English)