The plan:

December 19, 2014 Trunk is open for business. (Post 4.1)
January 21, 2015 Initial 4.2 meeting.
February 18, 2015 (+3w) Decision time for features being developed as a plugin. Merge window opens for any blessed for 4.2.
February 25, 2015 (+1w) Feature plugin merge window closes.
March 11, 2015 (+2w)
March 12, 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 4.3.
March 18, 2015 (+1w)
March 19, 2015
Beta 2.
March 25, 2015 (+1w)
March 26, 2015
Beta 3.
April 2, 2015 (+1w)
April 3, 2015
Beta 4.
April 8, 2015 (+1w)
April 15, 2015
Release candidate. (More release candidates as necessary.)
April 22, 2015 (+2w) Target date for release of WordPress 4.2.

To get involved in WordPress core development, head on over to Trac and pick a 4.2 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!