When does the blood flows through the coronary artery during left ventricular systole or right ventricular systole?

Neither, theoretically the two ventricles contract simultaneously. The coronary arteries begin as two holes just above the leaflets of the aortic valve. During systole the leaflets block the coronary arteries and prevent blood flow to them. It is during diastole (of both ventricles) that blood returns to the coronary vasculature.