On the Symfony Blog they've posted the official announcement of their Fourth Symfony Docs Hack Day happening on May 21st. This hack day is focused on just improving the documentation for the framework, not handling bugs in the main codebase itself.

The Symfony project is proud to announce its fourth Symfony Docs Hack Day. This Hack Day will be an online event to give a push to the Symfony Docs before the Symfony 3.1 release at the end of this month.

[...] Hosts Ryan Weaver, Wouter de Jong and Christian Flothmann along with you and all your friends from the Symfony community. The Hack Day is for everyone - we need Symfony experts and newcomers. If you're new to Symfony, you give us a fresh look at the documentation!

The post gives you a bit of an idea what the event will be like and what you can expect, especially as a first time submitter. It will be happening completely online via the "#symfony-docs" channel on the Freenode IRC network. You can prepare by following some of the links in the post to pending pull requests and a list of missing documentation contents.