The nightly build of Alfresco Community Edition is available at this URL
Alfresco has a large set of automated tests and various builds run continuously as and when updates to the source code are made. Should a build pass all tests then it will be uploaded to the 'downloads' and become available on the above URL. So a nightly build may from time to time be considerably older than 'last night'.
Nightly builds allow the earliest preview of new features, bug fixes and integration of community contributions.
Nightly builds are unlikely to be suitable for production use except during times when there is little change from a stable release.