It was really a long time ago for us, but it did kind of change everything in the galaxy. Before that everyone was mostly just stuck on planet, and like, within systems and things.
Life Day is a Wookiee holiday. It's sort of what it sounds like, it celebrates life - because their planet is kind of full of it, and those that have lived and been lost, and children, and just, well, life. It's just a celebration of those things.