*Halloween, as with many holidays, overlaps with Samhain,Feralia, and Pomona and leads up to All Souls Day and the Day of the Dead. It is a holiday that focuses on the changing of the seasons, the end of the harvest, fading of the light, death and the cycle of life, which death is of course a part.
*Some traditions believe that this is the time of year when the dead get to transition to the next phase, be it their place of rest or their next incarnation.
*Halloween's current position on our calendar, is a result of Pope Gregory III shuffling the Christian holdays (All Hallows and All Saints used to be celebrated in May).
*The idea of dressing up ghoulishly originated in Europe after outbreaks of the plague led to a lot of people having a lot of dead people to celebrate. In Celtic cultures, people dressed as animals to lead the dead (who they thought had been turned to animals) home.
*Trick-or treating is - depending on who you talk to - either a off shoot of the original Celtic hunt for soul cakes or a variation of the Guy Fawkes celebrations that shifted to a day that people were actually celebrating in the US. (Guy Fawkes Day is November 5th).
*Jack-o-lanterns used to be made with other vegetables, but they weren't prevalent in the US when the holiday caught on, so pumpkins were substituted.
*The candles - be they in pumpkins or not - are to help light the way for the dead. *Some culture's believe that god dies on Halloween and is reborn at Yule. (Which also coincides with the winter solstice and the days getting longer again.)
In whatever manner you choose, go forth and celebrate. (And remember holiday candy has no calories. For real!)
Thanks to belief.net and Wikipedia for helping me check my facts.