Systole occurs between s1 and s2 and after active filling fill 100% of the ventricles (caused by the sequential contraction created from the SA node). The increased pressure in the ventricles will cause the now lower pressured atrium valves to shut causing s1. Then with all valves closed this creates what is called an Isovolumetric contraction. This occurs when the SA node contraction will travel down the AV node which travels down the septum and eventually reaches the purkinje fibers and causes a simultaneous contraction and open the semilunar valves. This entire process occurs during the systolic phase.
An easy mnemonic to remember is diastolic which is the opposite of systolic sounds like it has the word "die" in it. Dying is when we usually relax and Diastole is the relaxation phase of the cardiac cycle. Systole would therefore be the contraction phase.