The FA Cup matches will take place at Wembley over the July 18-19 weekend with no fans present.
source https://www.bostonglobe.com/2020/06/28/sports/arsenal-manchester-united-chelsea-reach-fa-cup-semifinals/
source https://www.bostonglobe.com/2020/06/28/sports/arsenal-manchester-united-chelsea-reach-fa-cup-semifinals/