Rick Welch believes you should breathe in as you draw the bow, hold the breath as you shoot, and exhale when the arrow hits the target.
Another good archer on MBB III does just the opposite: he exhales as he draws and then I'm not sure what he does as he holds and shoots. Maybe he exhales half his breath and holds the remaining half while he shoots.
The point is, it is good to establish a consistent pattern, and stick with it shot after shot. Not much time to do that between now and this weekend!