Under Network > Staking > Targets we can see some validators are selected for the next session / active set.
https://polkadot.js.org/apps/?rpc=wss%3A%2F%2Fkusama-rpc.polkadot.io#/staking/targets
What event is this on the blockchain?
Can I listen for this, e.g. using rpc/js api?