https://wiki.ubuntu.com/NattyReleaseSchedule
2010/10/28 Developer Summit
2010/11/25 Feature Definition Freeze 2010/12/02 Alpha 1 2010/12/30 Debian Import Freeze 2011/02/03 Alpha 2 2011/02/24 Feature Freeze 2011/03/03 Alpha 3 2011/03/24 UI Freeze 2011/03/31 Beta 1 2011/04/14 Beta 2 & Final Freeze 2011/04/28 [COLOR="Red"]Ubuntu 11.04[/COLOR]