Origin of Phrases - A
At the eleventh hour
Meaning: To raise a problem, issue, or solution right before an important deadline.
Example: I had given up after trying for days to get tickets to a sold out show, then at the eleventh hour a friend calls to tell me he has tickets.
Origin: On a 12-hour clock (rather than the 24-hour clock used by scientists, the military, et al) the hours of 12 noon and 12 midnight seem to hold special significance. De-marking the transition from morning to afternoon and the end of the day, they are often used as deadlines (high noon, the stroke of midnight).
To come at "the eleventh hour" implies that it comes in the last hour before the deadline. The choice of "the eleventh minute of the eleventh hour of the eleventh day of the eleventh month" as the time to end W.W.I was quite apt.