The undertaking begins, as in He's going to announce his candidacy for mayor—the balloon goes up on Monday. This expression comes from World War I, when British artillery sent up a balloon to notify gunners to open fire, this visual signal being more reliable than courier or telephone. It was soon transferred to signal other kinds of beginning. 
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.