Typically linked to the Celtic festival of Samhain (pronounced sow-an or sow-in)", derived from the Old Irish Samuin meaning "summer's end". Samhain was the first and by far the most important of the four quarter days in the medieval Irish calendar and, falling on the last day of Autumn, it was a time for stock-taking and preparation for the cold winter months ahead. There was also a sense that this was the time of year when the physical and supernatural worlds were closest and magical things could happen. To ward off these spirits, the Irish built huge, symbolically regenerative bonfires and invoked the help of the gods through animal and perhaps even human sacrifice.
Halloween is also thought to have been heavily influenced by the Christian holy days of All Saints' Day (also known as Hallowmas, All Hallows, Hallowtide) and All Souls' Day. Falling on November 1st and 2nd respectively, collectively they were a time for honoring the Saints and praying for the recently departed who had yet to reach heaven. By the end of the 12th century they had become days of holy obligation across Europe and involved such traditions as ringing bells for the souls in purgatory and "souling", the custom of baking bread or soul cakes for "all crysten [christened] souls".
Oh, for zombies:
Or if you're feeling frisky, want that up-close-and-personal experience: