Simple.
Make sure it has at least 3 paragraphs and make sure to get details on how the day you choose was the best day of your life so far. Also, make sure you use words that are correct for the terms in which you are using them. If today is the best day of your life then use present-tense words. If it was in the past then use past-tense words. Make sure the story has an easy understanding, but don't make it too easy or it won't really seem like you put much effort forth to make the essay. When your teacher reads your essay she/he should want to be intrigued and would want to know all about it. Use a hook to make your essay stand out from the others that there may be. This is a useful strategy that makes people enjoy reading your essay.
I hope this helps you, and I hope you get a good grade.
-Sayonara