The plan:

April 23, 2015 Trunk is open for business. (Post-4.2)
April 29, 2015 Initial 4.3 meeting.
June 3, 2015 (+5w) Decision time for features being developed as a plugin. Merge window opens for any blessed for 4.3.
June 17, 2015 (+2w) Feature plugin merge window closes.
July 1, 2015 (+2w) 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 4.4.
July 8, 2015 (+1w) Beta 2.
July 15, 2015 (+1w) Beta 3.
July 22, 2015 (+1w) Beta 4.
July 29, 2015 (+1w) Release candidate and soft string freeze (More release candidates as necessary).
August 18, 2015 17:00 UTC (+3w) Target date for release of WordPress 4.3.

To get involved in WordPress core development, head on over to Trac and pick a 4.3 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!