I love Halloween, eating all that candy you get from trick-or-treating, and dressing up in costumes are two of my favorite activities.
I love Halloween too, How did it start as tradition of dressing up and collecting candy?
Well most believe the traditions started as ancient Celtic harvest festivals, which may have had pagan roots. I think the trick-or-treating was probably popularized by candy companies if I had to guess.
Seems like Pagans have some hand in our holidays even today
Definitely, there is dispute, though. Some people think that Halloween began solely as a Christian holiday separate from any sort of Pagan background.
But why the costumes and dressing up, to ward off evil spirits?
That seems like one possibility. Halloween is just one day of the three-day Allhallowtide holiday which involves remembering the dead, so maybe dressing up is a way to remember them? 
hmm three days, we should adopt that for our Halloween
Well Halloween doesn't last three days, the other two days are known as All Saints' Day and All Souls' Day, Halloween is referred to as All Saints' Eve in that version.
So Halloween is actually a festival but we turned into a costume, collecting candy holiday