When the heart muscle contracts (pushes in) it is called "systole". When the heart muscle relaxes (stops pushing in), this is called "diastole". Both atria do systole together. Both ventricles do systole together. But the atria do systole "before" the ventricles. Even though the "atrial systole" comes before "ventricular systole", all four chambers do diastole at the same time. This is called "cardiac diastole"