It was perhaps Jeremy Schwartz who said: "Live every day as if it were your last, because one of these days, it will be."
Many people regard it as a profound statement, with attributions to Mr Steve Jobs, Mr Muhammad Ali and to whoever they consider a man wise enough to make such a wise statement.
I found a rather funny criticism, nonetheless valid, of this statement here.
The latter part of the original sentence is no doubt true, but Mr Schwartz perhaps had no training in that obscure field of study known as Probability Theory.
You will die one day, but if the average life span of a human being is 70 years, then the probability that today is your last day is approximately 1 in 25,000. In fact the probability is much less than that if you consider that the days of death for humanity are not evenly distributed throughout the seventy years but are concentrated towards the very end. So, if you are below 40, it is not unreasonable to assume the probability of today being your last day to be 1 in 50,000.
Living today as if it it were your last day is therefore horribly bad advice. It is like betting all your money on a play of the roulette wheel, with the difference that the roulette wheel in this case has 50,000 slots.
I think the advice can be more useful if it is worded differently, but then it won't be that spectacular:
Live each day adhering to your highest principles. It may be your last day, so there may not be an opportunity to correct the wrongs, to apologize, to regret, to make amends, to counteract a harsh word with a loving one, and to kiss a cheek that you may have caused a tear on.
Live without deceit and with a clean conscience. Live so that your heart and mind does not accumulate baggage of hurts and regrets and of waiting for the right opportunity to do something good. It is never too early to do the right thing, whether it be to more giving and forgiving, to apologize and to make up, to start living a healthy life, to be charitable and generous, to be kind and loving, to keep in touch with people that you care about, ...
Make the best of every day. This day will never happen again. And therefore today is worth it. Today is the day you start to redeem yourself. Today is the day you execute on your promises to yourself. Today is the day you are alive, and therefore do not squander this day.
Make it count, towards what is meaningful to you.