Samhain (pronounced "So'win" or "so-wain") is the ancient Gaelic festival that celebrates the end of summer and the onset of winter. It's the Celtic new year and the day that the dead of the past year are honored.

In pre-Christian Britain (and even for some time after the arrival of the religion) various Celtic peoples celebrated this holiday, lighting bonfires, making offerings and sacrifices to the gods and spirits for a good year.

Now the festival has underwent much cosmetic surgery and redressing and is now own as Halloween, many of the traditions we have during it are simply altered practices from by-gone days. The idea of dressing up evolved from young men painting their faces black and running about startling others as a way to appease the spirits of the dead.
The jack o' lantern came from the tradition of carving turnips into frightening faces to scare away lost spirits from the home, due to the belief that since Samhain is also the day of the dead, and the god of the dead is gathering all of the spirits to cross over into the otherworld, and the veil that separates ours with their world is thin around this time.

It's very interesting to read about how our various holidays have evolved from pagan practices, and how we can glean a deeper meaning of the celebrations by knowing WHY we do them. And considering that most of our holidays now are so watered down that they have almost no semblance of their predecessor, so for a neo-pagan, it is always good to know the history of their beliefs.

Happy Samhain!