If you use iCal, you absolutely could do this! Create a calendar called "Archive" or some such and when the task is done, change its calendar to Archive. You might also put the previous context and any notes in the event, but unless you're a kGTD user, you probably don't have contexts delimited too granularly in you calendar so you likely did that anyway. So, yah, move the events to Archive and keep archive unchecked so you don't have to see it. I did a quick test of this and the only thing that doesn't work well is repeating events. It doesn't give you the option to just change the calendar for one instance like it does if you delete a recurring event instance. I suppose that someone with more Applescript chops than I have could whip up a script that would get the selected item, add its current calendar name to the notes, check if it is a recurring event, if so, create a copy in the Archive calendar and delete the original, otherwise just change the calendar of the original. Doesn't sound too tough to me, but I don't have time to get my hands dirty with it.