read Chapters 22,23 & 24 from this book : Kahneman, D., (2011), Thinking, Fast and Slow, Farrar, Straus and Giroux: New York ISBN 978-0-374-27563-1 (2013 Reprint:ISBN: 978-037453355-7)

Then please answer this question by 5 sentences no more :

Describe the ‘planning fallacy’ and the approach Kahneman suggests for mitigating it. please make it simple word,