Cardano (ADA) will be upgrading their blockchain from the "Byron" mainnet to the "Shelley" mainnet. The current Cardano Byron mainnet will become the Cardano Shelley mainnet on Wednesday, 29 July, 2020 at 21:44:51 UTC, starting at epoch 208.
Bittrex is working with the Cardano team to support the Shelley mainnet. Cardano wallets will be shut off for withdrawal and deposit while we work to complete the upgrade. Details on the timing of wallet maintenance will be shared once a time for the upgrade has been finalized.
Further information:
Announcement: https://forum.cardano.org/t/a-shelleysunday-update-and-some-exciting-news-icymi/36449
Roadmap: https://roadmap.cardano.org/en/shelley/
Maintenance Details: https://forum.cardano.org/t/cardano-foundation-s-communique-on-possible-exchange-interruptions/36697