It is open during ventricular diastole-ventricular diastole is the period when the ventricles relax and fill with blood. so, the av valves need to be open at this time so that blood can flow from the atria to the ventricles. the right av valve is called the tricuspid valve-it has three leaflets. the left av valve is called the mitral valve and has two leaflets. remember, the right side of the heart is pumping blood to the lungs so that it can be oxygenated. so, the blood flowing through the right av valve will be deoxygenated, while the blood flowing through the left av valve will be oxygenated.
finally, remember that the chordae tendineae and the papillary muscles do not pull the av valves open! These structures serve to prevent the valves from prolapsing during systole.