There is no tomorrow. If you want to do something, then you should do it today. Because, tomorrow, will always be a new day with new possibilities.
However, most of us try to make the most out of our times in a day by carrying some tasks of today to the next day. We do not realize that like this, we will be carrying all our undone tasks to a tomorrow that will never come, and these undone tasks will slowly be making a mountain and they will be waiting for them tomorrow that never comes.
And then one day, when finally, the tomorrow approaches, you will see how tall the peak of your undone tasks have become. Then you have to win it to progress in your life.
That is why the quote says that tomorrow is only found in the calendar of fools. This is why we should always complete our tasks every day. This way, even if you cannot manage to do some tasks today, it will not be a very big problem to complete the task in the next day.
Thus, always keep trying to complete the tasks when it is meant to be done, This way you will get the best use of the daily time you get, and this way you will grow as a better human being, who knows how to take responsibilities and maintain their commitments.