During the diastolic phase, do the ventricles relax and fill with blood.

Yes

Explanation: At the start of the cycle, during ventricular diastole–early, the heart relaxes and expands while receiving blood into both ventricles through both atria; then, near the end of ventricular diastole–late, the two atria begin to contract (atrial systole), and each atrium pumps blood into the ventricle 'below' it.