Drupal 8.8.0-alpha1 will be released the week of October 14th

In preparation for the minor release, Drupal 8.8.x will enter the alpha phase the week of October 14th, 2019. Core developers should plan to complete changes that are only allowed in minor releases prior to the alpha release. The 8.8.0-alpha1 deadline for most core patches is October 11. (More information on alpha and beta releases.)

• Developers and site owners can begin testing the alpha after its release.
• The 8.9.x and 9.0.x branches of core will be created, and future feature and API additions will be targeted against that branch instead of 8.8.x. All outstanding issues filed against 8.8.x will be automatically migrated to 8.9.x.
• Once 8.9.x is branched, new alpha experimental modules will be removed from the 8.8.x codebase (so their development will continue in 8.9.x only). The Help Topics and Config Environment modules are new alpha stability modules in 8.8.x.
• All issues filed against 8.7.x will then be migrated to 8.8.x, and subsequent bug reports should be targeted against the 8.8.x branch.
• During the alpha phase, core issues will be committed according to the following policy:
  1. Most issues that are allowed for patch releases will be committed to 8.8.x, 8.9.x, and 9.0.x.
  2. Most issues that are only allowed in minor releases will be committed to 8.9.x and 9.0.x only. A few strategic issues may be backported to 8.8.x, but only at committer discretion after the issue is fixed in 8.9.x (so leave them set to 8.9.x unless you are a committer), and only up until the beta deadline.

Drupal 8.8.0-beta1 will be released the week of November 4th

Roughly two weeks after the alpha release, the first beta release will be created. All the restrictions of the alpha release apply to beta releases as well. The release of the first beta is a firm deadline for all feature and API additions. Even if an issue is pending in the Reviewed & Tested by the Community (RTBC) queue when the commit freeze for the beta begins, it will be committed to the next minor release only.
The release candidate phase will begin the week of November 18th, and we will post further details at that time. See the summarized key dates in the release cycle, allowed changes during the Drupal 8 release cycle, and Drupal 8 backwards compatibility and internal API policy for more information.

Bugfixes and security support of Drupal 8.6.x and 8.7.x

Drupal 8 core provides security coverage for the previous minor release as well as the newest minor release. Accordingly, security releases for Drupal 8.7.x will be made available until June 4th when Drupal 8.9.0 is released. Bugfixes that are not security-related will only be committed until Drupal 8.7.x's final bugfix window on November 6th.
Normal bugfix support for Drupal 8.6.x ended in May 2019. However security support is provided for 8.6.x until the release of Drupal 8.8.0 on December 4th, 2019.Note: June 4, 2020 is both the scheduled release date of 8.9.0 and also the target release date for Drupal 9.0.0. If 9.0.0 misses the June window, its fallback release date is December 2, 2020. Support and security coverage for 8.7.x, 8.8.x, and 8.9.x will remain the same in either scenario.