It turns out before the launch of the main net will also snapshot?
Snapshot will be taken to ensure all balances from RC network will be moved to the mainnet.
but what do we need to do for this? (update client?)
the exact date I understand, too, will be known later?
No need to do anything. Just back up the wallet and wait for the mainnet release. All semux balance will be moved automatically to the mainnet after the release.