Hedera will be upgrading the Mainnet to v0.7.0 (release notes:
https://hedera.io/release-notes) on Tuesday, Sept 8th 2020 at 17:00 UTC. In addition, Mainnet node 8 (0.0.11) and Mainnet node 12 (0.0.15) will be brought back online, bringing the node count to 13/13.
This upgrade will take approximately 2 hours and 30 minutes to complete, during which time all network services will be taken offline.