a5e64d83_ureStatus20120119____XWiki_org__Status_as_of_2012_01_19
[Feature] Calendar (built-in calendar application) [Status as of 2012/01/19] 
Calendar app has been retired because its quality wasn't good enough. We need to decide whether we wish to create a Calendar app or let user use an external integration only.
Feature Status - Jan 2012 (Blog.FeatureStatus20120119) - XWiki.org
Status as of 2012/01/19