Katara always said that her water bending was stronger at night, stronger still during a full moon. That was why the final attack on the Fire Nation was at night. The fire benders were weaker while the water benders were stronger.
Sokka had not really listened to the explanations his sister gave about the moon giving her energy and power until after Yue had died. He understood. Now, whenever he fought at night, under the moon, he felt a burst of energy course through him. With that energy, he outran a deadly lightening attack and pierced Azula's heart.
As he wiped off the blood from his sword, he looked up to the moon to see Yue smiling down on him. "Thank you," he whispered to her before he headed back into the fray of battle, one he was sure could be won.