(This blog post is taken from an announcement by Neal Caidin the Sakai Community Coordinator)
Congratulations amazing worldwide community!
Our community wiki has both functional  and technical  release notes.
Download instructions are also available of course  .
Several of the top changes include:
- Improved look and feel. New skin. More attractive and functional.
- Improved responsive design, better performance on mobile devices.
- Gradebook – performance enhancements, plus a number of minor interface improvements such as progressive rendering of student grades for large classes, and a new feature which allows instructors to view and adjust final grade distribution.
- Assignments – Refactoring of Assignments tool. Makes Assignments more maintainable and easier to enhance.
- Lessons has a new optional subpage left menu navigation feature. In addition, a number of useful enhancements such as a forums widget, announcements widget, calendar widget, resources widget, and personalization of text.
- Test and Quizzes (Samigo) – Extended delivery of assessments for select individuals and groups.
- Chat tool – updated interface.
- Pluggable ContentReview which contains 4 plagiarism detection integrations.
- Standards – support for IMS Content Item, including a “shopping cart” ubiquitously available in the rich text editor.
- Accessibility improvements.
 Functional release notes – https://confluence.sakaiproject.org/display/DOC/Sakai+12+Release+Notes
 Technical Release notes – https://confluence.sakaiproject.org/display/DOC/Sakai+12+Technical+Release+Notes
 Download instructions – http://source.sakaiproject.org/release/12.0/