The plan:

August 19, 2015 Trunk is open for business. (Post-4.3)
August 19, 2015 Initial 4.4 meeting.
September 30, 2015 (+6w) Decision time for features being developed as a plugin. Merge window opens for any blessed for 4.4.
October 7, 2015 (+1w) Feature plugin merge window closes.
October 21, 2015 (+2w)
October 22, 2015
Beta 1.
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 4.5.
October 28, 2015 (+1w) Beta 2.
November 4, 2015 (+1w) Beta 3.
November 11, 2015 November 12, 2015 (+1w) Beta 4.
November 18, 2015 November 25, 2015 (+1w) Release candidate and soft string freeze (More release candidates as necessary).
December 8, 2015 (+3w) Target date for release of WordPress 4.4.

To get involved in WordPress core development, head on over to Trac and pick a 4.4 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!