Calendar is an homage to the classic Mac version of the old Calendar 2.1. It allows you to quickly store and search for notes that are attached to specific days. It's not a replacement for iCal, but allows you to make quick entries without having to go through a lot of steps. It's an example of Less-is-Moreware.
Mac OS X 10.3.9, English version only (although one may set the "Region" to United States in the International pref pane).