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
Allhallowtide
Allhallowtide, Hallowtide, Allsaintstide, or the Hallowmas season, is the triduum encompassing the Western Christian observances of All Saints' Eve (Halloween), All Saints' Day (All Hallows') and All Souls' Day, which last from 31 October to 2 November annually. Allhallowtide is a "time to remember the dead, including martyrs, saints, and all faithful departed Christians." The present date of Hallowmas (All Saints' Day) and thus also of its vigil (Hallowe'en) was established for
many Halloween traditions originated from ancient Celtic harvest festivals, particularly the Gaelic festival Samhain; that such festivals may have had pagan roots; and that Samhain itself was Christianized as Halloween by the early Church. Some believe, however, that Halloween began solely as a Christian holiday, separate from ancient festivals like Samhain.
Halloween activities include trick-or-treating (or the related guising and souling), attending Halloween costume parties, carving pumpkins into jack-o'-lanterns, lighting bonfires, apple bobbing, divination games, playing pranks