Symfony manages its releases through a time-based model. A new Symfony minor version comes out every six months, one in May and one in November. Check out the release process details.
Latest Symfony versions
Maintained Symfony branches
These are the Symfony branches currently supported for bug fixes. Check out the Symfony release process for the full details.
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Symfony version checker
Jul 2011 Release
Mar 2013 End of support for bug fixes
Sep 2013 End of support for security fixes