The official Ubuntu 11.10 release schedule is available from the Ubuntu Wiki with the various release dates.
The first alpha release of Ubuntu 11.10 is planned for the 2nd of June, the second alpha release is planned for the 30th of June, and a third Alpha release will not come until the 4th of August. After a quiet July (though Ubuntu 10.04.3 LTS is set for the 14th of July), the feature freeze is happening on the 11th of August followed by the user-interface freeze two weeks later. This also marks the beta freeze.
The first beta will come on the 1st of September while the second and last beta will come on the 22nd of September during Oktoberfest. The kernel freeze is happening a week before that point. Lastly, a release candidate is set for the 6th of October and the official release of Ubuntu 11.10 is written in for the 13th of October.