Official releases are usually created when the developers feel there are sufficient changes, improvements and bug fixes to warrant a release. Due to the voluntary nature of Solr, no releases are scheduled in advance.
Both binary and source releases for the latest version of Solr are available from the Apache Mirrors.
Archives for all past versions of Solr are available at the Apache archives.
The solr-VERSION.zip or .tgz (where VERSION is the version number of the release, e.g. 3.0.1) file contains Apache Solr, html documentation and tutorial. The solr-VERSION-src.zip or .tgz contains the full source code for that version.
Note: prior to the Solr 4.1 release, all distribution and artifact filenames contained an "apache-" prefix.