Is it important to know how everyone else remembers an event before you write about it?

Yes, you want to have the most accurate information possible when writing about something that occurred so you don't look foolish and everyone would be on your side about the event.

Yes, and the reason why is because you need to know the mood of how to write about it. In other words, if everyone remembers this event as a really sad day, then  you would write about it as a tragedy, using words like unfortunate. But if it was a really happy day, then in your writing you would use words like fantastic.