This date was a pivotal time of the year; when the seasons changed. However, more importantly, observers also believed the boundary between this world and the next became especially thin at this time, enabling them to connect with the dead. This belief is shared by some other cultures; a similar idea is mentioned around the Jewish holiday of Yom Kippur, which also typically occurs in October and involves saying prayers for the dead. This is where Halloween gained its "haunted" them from.
The word "Halloween" dates back to around 1745 and comes from All Hallows' Eve and means ‘hallowed evening.’ Hundreds of years ago people dressed up as saints and went from door-to-door reciting songs. Children would also go door to door asking for soul cakes, a treat similar to biscuits, which is the origin of Halloween costumes and trick-or-treating.
Over the years as Christianity took over and the pagan undertones of the holiday were lessened, some basic traditions and elements of the holiday remained a part of our culture and simply evolved becoming what we now know and love as Halloween.