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