The plan:

January 15, 2014 Initial 3.9 meeting. Trunk is open for business.
January 29, 2014 Decision time for features being developed as a plugin. Merge window opens for any blessed for 3.9.
February 12, 2014
February 19, 2014
Feature plugin merge window closes.
February 26, 2014
March 5, 2014
March 11, 2014
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 4.0.
March 12, 2014
March 20, 2014
Beta 2.
March 19, 2014
March 29, 2014
Beta 3.
March 26, 2014
April 8, 2013
Release candidate. (More release candidates as necessary.)
April 16 Target date for release of WordPress 3.9.

To get involved in WordPress core development, head on over to Trac and pick a 3.9 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!