All Hallow's Eve is the night before All Saints Day. Over the years, the name has been shortened to Hallowe'en (from Hallow Even).
In Northern Europe, Samhain starts on October 31 and ends on November 1. This was the time of the season designated to honor the bountiful harvest, and to ask for a kind winter. It was also a time to honor the Ancestors' spirits for helping to create the abundance in the family.
Since the celebration started on the 31st by our current calendar, this is the time that the Ancestor Spirits started to come into this plane. Since some spirits can come in with ANGER because of what the family has done, or by what was done to them, we have the tradition of creepy, scary spirits. Where the family has treated the Ancestors with COMPASSION, love, and where they have communicated with the Ancestors with prayers and offerings, the spirits come in as benevolent.
The idea of scary faces on pumpkins was to scare off the spirits that were roaming the Earth that were not related to you, and who may do harm.
Tonight is a good night to remember the Ancestors with kindness and honor them.