I would open it, and then ignore it. How could someone tell you when you were going to die? Even a Dr that has diagnosed you with a terminal illness can give his best guess based on your condition and the state of your health, but he can't say exactly when and at what exact day you will die. If I was an overly anxious person, that worried to excess, I would not open it. The reason for that would not be because it would be true, but it could cause a great deal of anxiety and un-necessary worry in a person. I personally think only God knows exactly when we will die. Unless the envelope was from God, I would not worry about it, and I don't think he is sending out envelopes at this time with our expiration dates.
Assuming some miracle where the contents of the envelope was in fact true ....
I'm of a mixed mind in this case. On one hand, I wouldnt want to know, because I tend to get depressed easily and that sort of news would certainly put a huge downer (other than the obvious!) on my remaining time and I'd have one heck of a time making the most of that time i had left. I'd have a hard time not curling up in my bed in depressive state and wasting precious time. On the other hand, it would be beneficial, in that I would (hopefully) have time to de-clutter my life and do a few of the things on my bucket list, while ensuring everything is in order so that my kids and cats were taken care of afterwards. See ~ mixed mind. P.S I'm also a procrastinator and therefore hate making decisions without heaps of research and consideration of possible outcomes LoL!
"Cowards die many times before their death; The valiant only taste of death but once." - Julius Caesar, William Shakespeare. I shall tear up the unopened envelope, and consign it to the flames.