The plan (dates in UTC):

December 9, 2015 Trunktrunk A directory in Subversion containing the latest development code in preparation for the next major release cycle. If you are running "trunk", then you are on the latest revision. is open for business. (Post-4.4)
January 6, 2016 4.5 Kickoff meeting.
February 10, 2016 (+5w) Decision time for features being developed as a pluginPlugin A plugin is a piece of software containing a group of functions that can be added to a WordPress website. They can extend functionality or add new features to your WordPress websites. WordPress plugins are written in the PHP programming language and integrate seamlessly with WordPress. These can be free in the Plugin Directory or can be cost-based plugin from a third-party. Merge window opens for any blessed for 4.5.
February 17, 2016 (+1w) Feature pluginFeature Plugin A plugin that was created with the intention of eventually being proposed for inclusion in WordPress Core. See Features as Plugins. merge window closes.
February 25, 2016 (+1w) BetaBeta A pre-release of software that is given out to a large group of users to trial under real conditions. Beta versions have gone through alpha testing in-house and are generally fairly close in look, feel and function to the final product; however, design changes often occur as part of the process. 1. (Release on February 24 PST)
From this point on, no more commits for any new enhancements or feature requests in this release cycle, only bugbug A bug is an error or unexpected result. Performance improvements, code optimization, and are considered enhancements, not defects. After feature freeze, only bugs are dealt with, with regressions (adverse changes from the previous version) being the highest priority. fixes and inline documentation. Work can continue on enhancements/feature requests not completed and committed by this point, and can be picked up for commit again in just a few weeks at the start of WordPress 4.6.
March 3, 2016 (+1w) Beta 2. (Release on March 2 PST)
March 10, 2016 (+1w) Beta 3. (Release on March 9 PST)
March 17, 2016 (+1w) Beta 4. (Release on March 16 PST)
March 24, 2016 (+1w) Release candidaterelease candidate One of the final stages in the version release cycle, this version signals the potential to be a final release to the public. Also see alpha (beta). and soft string freeze (More release candidates as necessary). (Release on March 23 PST)
April 10, 2016 Release candidate 2.
April 12, 2016 (+3w) Target date for release of WordPress 4.5.

To get involved in WordPress coreCore Core is the set of software required to run WordPress. The Core Development Team builds WordPress. development, head on over to TracTrac An open source project by Edgewall Software that serves as a bug tracker and project management tool for WordPress. and pick a 4.5 ticket. Need help? Check out the Core Contributor Handbook. Get your patches done and submitted as soon as possible, then drum up people to test the patches and leave feedback on the ticketticket Created for both bug reports and feature development on the bug tracker.. Patches for enhancements won’t be committed after the posted dates, so that we can all focus on squashing bugs and hopefully deliver the most bug-free WordPress to date. Wish us luck!