The boom from the cannon reverberates over the field

  
There is something about a cannon blasting when it is your son's last field march before he graduates.  It was a touching experience as I look back and remember the flood of emotions: pride, nostalgia for an end of a chapter, nervousness because of the many hats I had to wear, and then knowing that this was a happy day for my son, and just to be there to support him.  What was that blast signalling?