Final answer:
Ventricular filling begins in the diastole phase of the cardiac cycle, which is the relaxation stage allowing the ventricles to fill with blood, followed by atrial systole for complete filling.
Step-by-step explanation:
The phase of the cardiac cycle during which ventricular filling begins is called diastole. This is referred to as the filling stage because during diastole, the ventricles are relaxed and fill with blood from the atria. Initially, the ventricles fill passively with about 70-80 percent of their capacity, after which atrial contraction (atrial systole) completes the remaining 20-30 percent of ventricular filling with what's known as the "atrial kick". Diastole is therefore essential for ensuring the ventricles are adequately filled with blood before they contract and pump blood out of the heart during systole.