You can edit the `sidebar.php` (or any `sidebar-xxx.php` file, depends on your theme) and interface with your database using the [wpdb class][1].

If your historical events are in the same DB as wordpress you can use the wpdb class straight away, if they are in another DB you can instanciate a new wpdb class with :

    $my_db = new wpdb('user','pass','dbname','host');


  [1]: http://codex.wordpress.org/Class_Reference/wpdb