The plan (dates in UTC):

December 9, 2015 Trunk 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 plugin. Merge window opens for any blessed for 4.5.
February 17, 2016 (+1w) Feature plugin merge window closes.
February 25, 2016 (+1w) Beta 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 bug 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 candidate 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 core development, head on over to Trac 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 ticket. 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!