If you have a website that maintains a list of events for users, it is a great idea to allow users to selectively add those events to their own calendar. Using automation from a website with something like Outlook is a bad idea. It would be blocked by the browser's security and your users might use something else for their calendar. Fortunately, many main-stream (most?) calendar applications, such as Outlook, Windows Calendar on Vista, and a whole lot more, support the iCalendar specification.