I know of a calandar to keep track of things like appoitments, due dates, birthdays or just anything worth marking down on a calandar, at http://codegrrl.com/scripts/phpcalendar/
Though having something similar to that incorportated into wordpress would be pretty darn cool!
Unfortunately, just as a warning, the codegrrl.com phpCalendar script can cause a conflict with WordPress, which results in the archives month links not functioning correctly.
It doesn’t seem to happen for everyone, but in my case, it did.
I have no clue how to begin to fix this, but apparently the two things are not necessarily compatible. 🙂
You could always try it, though, and if it works for you, that’s all that matters! 😀
I have a Christmas one made out of icing, red hots, and gum drops… oh, you said “editable” – I doubt if that will be incorporated into WP but you could write a hack. The trouble with doing it in WP is … well WP is a publishing system afterall, not an event scheduler. But like I said, you could hack it to display posts “x number of days” in advance. This has been discussed before (and probably someone has hacked up a version) so give a thorough search of the forum.