Hallow's End is a Halloween-themed event that takes place at the middle to end of October.
The day was once marked as a festival to celebrate the end of the harvest for the humans of Lordaeron; the Forsaken later adopted this practice in their own ways, and now use the event for their mysterious "Wickerman" rituals.
Hallow's End marks the day the group of undead, who would later be known as the Forsaken, broke free from the Lich King's dominance and regained their free will. They celebrate this occasion by setting fire to a Wickerman effigy and smearing the ashes across their faces. Sylvanas Windrunner, the leader of the Forsaken, also gives a short speech in commemoration of this ritual.
In the Warcraft book Arthas: Rise of the Lich King, it is revealed that Hallow's End was celebrated by the Alliance long before the Scourge devastated Lordaeron.