What is Dia De Los Muertos?

I have a Spanish cl & my teacher wants us to make something for The Day of The Dead, so i drew a skull with a lot of colors on it. :] and now we have to write a paper about it. We have to write about when it takes place, why it takes place and what do the people who celebrate it believe. We didnt even study this and every time I try to look it up i get diffrent answers. So now I'm confused. Helppp mee pleee.