Its spelt Halloween. 

October 31st is also a very important day for Christians because it celebrates Reformation Day

Do you mean October 31st? Holloween?

Just to add to what others have already said...in some cultures it is seen as the day when the world of the dead and the world of the living converge, so it is easier to contact the dead.  This originally was a a pagan celebration and used as a way of celebrating and contact loved ones who had died.  In some countries like in Latin America they still celebrate the "Day of the Dead" around this time. 

It has nothing to do with "evil" or anything bad.  It is thought that religious institutions declared them as "evil" since the celebration comes from paganism, but that is controversial.  There are lots of online resources with good information about this topic.

Ok. Thank you very much for your answers. They're so helpful.