Ventricular systole coincides with

The atria undergoes relaxation (diastole), coinciding with the ventricular systole. 

Ventricular systole increases the ventricular pressure causing the closure of tricuspid and bicuspid valves due to attempted backflow of blood into the atria. As the ventricular pressure increases further, the semilunar valves guarding the pulmonary artery (right side) and the aorta (left side) are forced open. It allows the blood in the ventricles to flow through pulmonary trunk and aorta into the circulatory pathways.