A sun is when the diabolo does a complete revelation around the sticks. To do a sun all you have to do is swing the diabolo to your left al the way around so that it goes over both sticks and then falls down below both sticks. The downside of doing a sun this way is that the string gets crossed, to uncross the string just do a sun in the opposite direction, that would be to your right.
To keep the strings from getting crossed when you do suns you need to uncross the sticks as the sun is happening. To do this put your right stick under your left, hold that stick with your pinky, ring, and middle finger. The reach over the left stick and grab the right stick on the other side of the cross with your right hand pointer finger. Then do a sun, and as the diabolo comes around you let go of the right stick with your pinky, ring, and middle fingers so that you only hold on with your pointer finger.