The tail end of an Army saying when speaking about how many days are left until a certain date, usually how many days left in the field or left in a course. I think the saying may have been made popular by a movie or something, though I don't recall which one, if I even ever knew.

Typical usage would be something like:

"How many days left until we get to go back into base camp?"
"Twenty-seven and a wake up."

When I took the Canadian Army's junior leadership course for non-commissioned members (NCM's) a couple of years ago, I was the official keeper of the time left, so my job was to make sure I always knew how many days were left. It was kinda a moral thing, even when the days left were still in the sixties... but when we got to single digits, let me tell ya it was starting to look all good.