Death has always been something that came to you — naturally or via some agency — and not the other way around; and only rarely is its time known beforehand (i.e., executions). It is a relatively modern phenomenon that we actively seek death in these cases, and decide when to go whilst being a part of the community around you. Sure there are the customs of elderly tribesmen wondering off into the wilderness to die, but that too abstracts death away into something that happens to you, not something you actively invite.

Society isn't used to this notion, and we don't really have any handholds to fall back to. Do you mourn with the person while he is still alive? Go on as if nothing really changed? Celebrate life with them while you still can? Euthanasia is becoming widely accepted; at least at the abstract levels of law and medicine, so we'll probably figure it out eventually.

Perhaps the Hallmarks of the world can introduce a nice line of cards for the occasion?