The best known of all Pagan holidays, though most people know it as Halloween rather than Samhain (which is pronounced SOW-en, by the way). And no, there is no Celtic death God named Samhain. That's just a myth.
In the Wheel of the Year, this is the day the ageing God dies and the Goddess mourns him, bringing the cold of winter in her sadness. No worries, He'll be reborn again at Yule. It's also the last of 3 harvest festivals, representing the last of summer's bounty.
It's the day when the veil between the worlds is at it's thinnest ,and spirits are more likely to walk the Earth. It's a time for magick and mystery, which is why it's the most popular Sabbat among Pagans. If you're looking for more, I do have a larger section of pages just on Samhain worth noting.
The name "Halloween" comes from the Christian attempt to draw the Pagans away from their holidays. They created All Saints Day on November 1st, and the night before came to be known as All Hallow's Eve. Even with the attempted Christianization, the original Pagan activities won out.
Other names: Halloween, All Hallow's Eve
Traditions and activities: trick or treating, wearing costumes, carving pumpkins, divination, honoring the deceased
Correspondences: mugwort, patchouli, sage, orange, black
Foods served: pumpkins, apples, wine, corn, squash, colcannon, nuts, mead or ale