I frequently want to add multiple URLs to Calendar events. Of course, you can put them in the Notes section, but given that there’s a URL field, it seems a little kludgey. This solution is kludgey too, but perhaps a bit less so.

Drag the additional link(s) to the Finder to create a .webloc file; then drag that file to the attachment field for the event.

You can double-click the file to open the link, which is better than the link being non-clickable in the Notes field, where you would have to highlight and right-click (Control+click). The URL won’t appear in the body of an email when you send an event to someone, but it will be in the attached .ics file.

[crarko adds: I haven’t tested this one.]

Source: Mac OSX Hints