During the mainnet beta period, it is expected that the network will experience very brief periods of scheduled downtime for maintenance. 

Anticipated maintenance activities include adding additional nodes to the network and updating the core software running on mainnet (beta) nodes. The network’s consensus state will remain unchanged in the case of any downtime or network failures, and any transactions sent to the network during periods of downtime will be rejected.
 
The status of Hedera’s mainnet and testnets is publicly available at https://status.hedera.com/