If the balloons go up, North Korea promises a merciless military strike.  According to AP (via AZ central):

“Merciless military strike by the Western Front will be put into practice without warning” if South Korean activists make a move to fly leaflets on Monday, the North’s military said in a statement in English. It also warned South Korean residents in the border area to evacuate in advance.

In South Korea, Defense Minister Kim Kwan-jin said at a parliamentary hearing that his troops would “thoroughly annihilate” any base responsible for the strike if the North attacked.

Choe Sang-hun with New York Times adds:

An umbrella group of anti-North Korean groups, led mostly by defectors from the North, says it plans to release balloons carrying propaganda leaflets on Monday in Imjingak, near the border with North Korea. Activists have conducted similar balloon launchings before. The North’s official Korean Central News Agency said, “The leaflets are a most undisguised act of psychological warfare, a violation of the armistice and an intolerable act of war.”

I am assuming that Park Sang-hak is involved.  Balloon launchings are not new and have been discussed before on The Hole. See Dollars from Heaven and Park Sang Hak and North Korea threatens border closing and South Korean socks.