The following schedule is subject to adjustment, but reflects the current thoughts on achievable milestones.

January 02-09, 2013 Confirm proposed scope and secure team leads.
January 09, 2013 Development begins.
April 04, 2013 Hard freeze on feature development. Stop feature development; focus on testing and compatibility (supported platforms, browsers, RTL, accessibility, etc). Ship a Beta 1.
From this point on, no more new feature requests in this release cycle, only fixes for issues in current features.
April 29, 2013 Beta 2
May 03, 2013 Beta 3
May 08, 2013 Release Candidate 1 target date. String freeze. Any work is focused on regressions and blockers only.
May 15, 2013 Release Candidate 2
May 20, 2013 Target date for WordPress 3.6 launch.

To get involved in WordPress core development, head on over to Trac and pick a 3.6 ticket. Need help? Check out the Core Contributor Handbook.