Niantic has announced besides 100 sites in each phase of Kythera anomaly series, 10 cities are selected to have shards during May 21 14:30 - 17:30 local time:
Shizuoka City, Shizuoka, Japan
Dubai, UAE
Denizli, Turkey
Kraków, Poland
Stuttgart, Germany
Cambridge, UK
La Plata, Argentina
Grand Rapids, MI, USA
Midland, TX, USA
Tapachula, Chiapas, México
For more info like rules and shards timeline, see https://community.ingress.com/en/discussion/19131/kythera-phase-2
