Early 2021.
Because the game is reusing Breath of the Wild's engine and assets, that cuts out a good chunk of the development time. Because of that, I think they're already much farther along with this game than compared to how far they were with Breath of the Wild when it was first revealed at E3 2014.
Even though they didn't announce a date, I think that the development team is internally targeting a Holiday 2020 release. But, I expect it to be delayed, because it's 3D Zelda. It always gets delayed. That, and I believe the next 3D Mario game will be the big holiday title for 2020.
Hardware Comparison Threads:
Current Thread
Is Hardware Getting TOO Powerful?
Older Threads:
PlayStation/Xbox/Switch: 2022 Edition
PlayStation/Xbox/Switch Hardware Battle: 2021 Edition!
PlayStation 4/Xbox One/Nintendo Switch: 2019 vs. 2020
PlayStation 4/Xbox One/Nintendo Switch: 2018 vs. 2019
PlayStation 4/Xbox One/Nintendo Switch: 2017 vs. 2018
PlayStation 4: 2015 vs. 2016 vs. 2017