Interesting.
Making a Calendar entry creates a post on the forum you choose, and adds the event to the calendar as well. I just created (then deleted) an entry for the Trumpeter Salute 2011 convention starting this Friday, so yes, anyone can add events to the Calendar.
You can't specify a range of dates, though, so you can't say "Convention X is happening on these three days", you can really only specify the starting date. Adding a second Calendar entry with the ending date on it will create a second post.
Would be a good way to begin convention threads here, and possibly "get your entries in by [deadline]" reminder threads for LPL and other contests... Even for those who don't pay any attention to the Calendar, the fact that it spawns normal forum threads too will remind us.