The calendar is as follows:
- November 5: transition freeze
- December 5: mandatory 10-day migrations
- January 5: soft freeze
- February 5: full freeze
The transition freeze means that library changes which introduce transitions are no longer possible (at least, not without convincing the release managers); this basically means that library changes which break the ABI and therefore need an soname bump can no longer get into testing (and therefore the next release).
Mandatory 10-day migrations means that all package migrations from unstable to testing will take ten days (up from the default of five, and possible 2-day migrations for "urgency=high" uploads). This leaves more time for issues to be found with new or updated packages in unstable before they migrate to testing (and therefore the next release).
The soft freeze means the contents of the distribution are fixed: no new packages can migrate to testing, and packages removed from testing cannot be re-introduced.
The full freeze means that packages no longer migrate automatically from unstable to testing; all migrations must be approved by the release managers. The freeze policy for Stretch is quite detailed; basically it only allows migrations from unstable to testing which are targeted bug fixes (for sufficiently important bugs), along with translation and documentation updates.
Until the full freeze, users of testing (only) shouldn't be particularly affected; all that should change will be that some library updates won't happen, and after the soft freeze, no new software will be available. Once in the full freeze, there will be fewer updates.
The biggest issues tend to affect users attempting to install new testing systems during the freeze: package removals tend to occur more frequently, and a package which is removed can't be installed any more. Pre-existing testing users are less affected by this: packages they already have installed won't be forcibly uninstalled, they just won't be able to re-install them. (Since transitions will be frozen, forced upgrades/removals related to library transitions won't be an issue.)
All the usual caveats related to running testing only apply, even more so; for example security fixes will take longer to migrate to testing. Any issues you do encounter though will be of interest to the relevant maintainers and the release managers: unless they're fixed, they'll affect the release... (But removing a package is a valid fix in many cases, which may not be to your liking!)