It would be awesome if we devoted 1 entire day to doing nothing but documenting MediaWiki. There are countless classes and functions in MediaWiki core and its extensions that have no documentation. There is also a serious lack of high level documentation for developers. The problem is that documentation is often an after-thought and rarely gets serious attention from developers.
Some suggested projects that could be part of MediaWiki Documentation Day:
* Tackle some of the blocking tasks at T2001.
* Add in-code function descriptions for important functions that don't have them.
* Add in-code class descriptions to classes that don't have them.
* Create documentation pages on MediaWiki.org for important classes in core that don't have them. See [[ https://www.mediawiki.org/wiki/Manual:User.php | Manual:User.php ]] for an example of a page that does exist. High-level documentation, like [[ https://www.mediawiki.org/wiki/Manual:Title.php#Title_structure | Manual:Title.php#Title_structure ]] is especially useful.
* Clean up our [[ https://www.mediawiki.org/wiki/Category:Outdated_pages | outdated documentation ]] on MediaWiki.org.
* Create README files for all the extensions that don't have them and make sure that all configuration variables are documented there.
* Create some high-level documentation on how to write new API modules.