"And it came to pass on that very day" (Exodus 12:51). "Etzem" ["very," literally "bone"] means nothing other than the strength of the day [the height of midday]. "The LORD brought the children of Israel out of the land of Egypt by their hosts" — this teaches that they did not go out in tumult and disorder, but in martial array [as an ordered army].
On the Strength of That Very Day Israel Left Egypt in Battle Array
Mekhilta DeRabbi Shimon Ben Yochai 12:51
ויהי בעצם היום הזה אין עצם אלא תקפו של יום: הוציא ה' את בני ישראל מארץ מצרים על צבאותם. מלמד שלא יצאו מהומה אלא מטורקסין: