When atria are contracting ,the atrioventricular valves open. So, do the semilunar valves open or close?

Answer 1

AV valves let the atria fill the ventricles. The semilunar valves let the ventricles "fill the body" so to speak. It doesn't work if they're all opened at the same time. AV open, Semilunar closed and vice versa

Answer 2

The semilunar valves close shortly before the atrioventricular valves open

End of isovolumetric contraction when the semilunar valves close. This is associated with the second heart sound and so at this stage all valves closed as the atrioventricular valves close first with the first heart sound. (Phase 5)
http://www.cvphysiology.com/Heart%20Disease/HD002e.htm

The atrioventricular valves are open between phases 6 to 1.
http://www.cvphysiology.com/Heart%20Disease/HD002a.htm

Answer 3

The semi lunar valves are closed, otherwise the ventricles would fill with blood reversing back through the aorta.