Well, you're supposed to get a day off (Monday after Easter), so it's happy. And, yes we're celebrating Jesus' victory against death aka ressurection, in the hopes that when we die our souls will be ressurected and keep on existing and living happily ever after (eternally) on the other world.
Imagine you can live eternally and play UrT (for example
) eternally. The thought of something like that is to be celebrated.