The facts about the important elements of a situation constrain and enable actions. These facts define the planning situation and the circumstances associated with implementing the plan. Favorable circumstances refers to situational factors that can contribute to attaining a specific goal.

"While heeding the profit of my counsel, avail yourself also of any helpful circumstances over and beyond the ordinary rules. According as circumstances are favorable, one should modify one's plans." - Sun Tzu

"Make your plans to fit the circumstances." - General George S. Patton, Jr

from Sun Tzu, The Art of War, Chapter 1
http://www.military-quotes.com/