The plan:

April 13, 2016 Trunk is open for business. (Post-4.5)
April 20, 2016 4.6 Kickoff meeting.
June 8, 2016 (+7w) Final decision time for feature projects. Merge window opens for any blessed for 4.6. (Note that feature projects can be approved and committed at any time during the cycle.)
June 15, 2016 (+1w) Feature projects merge window closes.
June 29, 2016 (+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 4.7.
July 6, 2016 (+1w) Beta 2.
July 13, 2016 (+1w) Beta 3.
July 20, 2016 (+1w) Beta 4.
July 27, 2016 (+1w) Release candidate and soft string freeze (More release candidates as necessary).
August 3 4, 2016 (+1w) Slot for a release candidate.
August 10, 2016 (+1w) Slot for a release candidate and hard string freeze.
August 12, 2016 (+2d) Slot for a final release candidate.
August 15, 2016 (+3d) Dry run for release of WordPress 4.6.
August 16, 2016 (+1d) Target date for release of WordPress 4.6.

To get involved in WordPress core development, head on over to Trac and pick a 4.6 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!