It's pronounced sauwin - Samhain is the old Irish spelling. There are eight holidays in the Wiccan religion. They're called Sabbats. The four main ones are the solstices and the equinoxes. Samhain is one of the cross sabbats coming between the fall equinox and the winter solstice. It is the time when the veil between the worlds is the thinest meaning spirits can travel back and forth and has been adapted by Christians as All Saints Day and the Mexicans as the Day of the Dead and by Americans as a fun time to wear costumes and carve pumpkins and play trick or treat. It used to be if you didn't get a treat, beware you might find some damage done to your property. Times have changed. Now you have to watch out for the tricks in the treats.
Now I guess I should try to remember the other three cross sabbats. I really didn't study Wicca very long and of course became disenchanted with it as I did with all organized religions. What I liked most was "Do as thou wilt but harm none" and I liked calling down one of the many (symbolic) Goddesses to assist me when I needed her help. Let's see there Lammas, that comes between the summer solstice and the fall equinox, probably some sort of harvest celebration. Beltane is the real fun one where you jump across the fire and go into the woods with a comvenient male for procreation. That's become May Day and comes between the spring equinox and the summer solstice. The fourth one is the one I always have trouble remembering. It's February 2nd. Imbolc I think. Now it's become Ground Hog's Day.
If you want to find out more, just Google Wicca.