The Debian devs are currently considering the following dates as freeze dates.
There not totally set in stone yet, but don't expect to change them.
2021-01-12 - Milestone 1 - Transition and (build-)essentials freeze
2021-02-12 - Milestone 2 - Soft Freeze
2021-03-12 - Milestone 3 - Hard Freeze - for key packages and packages without autopkgtests
To be announced - Milestone 4 - Full Freeze
https://lists.debian.org/debian-devel-announce/2020/03/msg00002.html