@Pepijn Sweep,
If I would somehow know that I will certainly (or even merely
likely) die tomorrow, I would notify the people I am close to, thank each for everything I feel they should be thanked for (starting with the person most important to me), then apologize to any I feel I should still apologize to, and then make any other comments or wishes. I would then decide on any final wishes I may have, and finally fix up some minor things that will make my death easier for others: make my computer less cryptic is all that comes to mind. Once all that is done, I would spend the rest of the time with a close friend or two as a good-bye party, but I wouldn't do anything I wouldn't do normally: probably no alcohol, even.