Samhain
Samhain (pronounced Sowen, or Sow-een) is probably the best known Pagan holiday, though most know it as Halloween. I'm sure you already know it's October 31st, and the third harvest festival of the year.
As with most old Pagan holidays, the Christian church attempted to overlay their own beliefs over the traditional ones in order to push people towards converting. The term "Halloween" comes from the Catholic holiday of All Saints Day (Nov 1st). The night before became known as All Hallows Eve, and it just got shortened to Halloween. Though the Catholic holiday still exists, the older Pagan celebration has won out in popularity.
On this day, it's said that the veil between this world and the astral realm is at its thinnest, allowing for spirits to travel among us. Though negative beings may also be afoot, this is not a day of evil or hostile 'ghosts'.
The aging God dies at Samhain, and the Goddess mourns His death until He is reborn at Yule. Her sadness is seen as the darkening days and cold weather.