hello, check this post : http://groups.google.com/group/rubyonrails-talk/browse_thread/thread/921051b1844a1ddd/93cfa15e7c907e00?lnk=gst&q=month#93cfa15e7c907e00 almost the same thing you want to do (except it is per monthes not per days).
your query should look like it: Post.find :all, :group => "DATE_FORMAT(date,'%Y-%m-%d')"
i'm not sure about DATE_FORMAT synthax, it is a mysql function but psql user you must check it.