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 Lucene, no releases are scheduled in advance. System Requirements are detailed here.
Both binary and source releases for the latest version of Lucene are available from the Apache Mirrors.
Archives for all past versions of Lucene are available at the Apache archives.
The lucene-VERSION.zip or .tar.gz (where VERSION is the version number of the release, e.g. 3.0.1) file contains the lucene-core jar file, html documentation, a demo application (see the "Getting Started" section) and various jar files containing contributed code. The lucene-VERSION-src.zip or .tar.gz contains the full source code for that version.