Samhain is one of the known Irish Celtic fire festivals. Throughout history, this holiday has been associated with ancestors and honoring the dead in one form or another across many cultures, especially Indo-European cultures. As ADF druids, we regularly work with the Ancestor Kindred through the Well Portal in each devotional ritual but in ancient times there were High Days specifically for them. Samhain is seen as a liminal time when the veil between the world in which we walk and the world of magic, the otherworld were opened and the Sidhe (faerie) folk and the Dead were close enough or able to walk among us.
Samhain is the end of the growing season and a noticeable point in the dying season as one can see by looking at the leaves change colors and fall. The frost begins to creep in and slowly kill the vegetation and animals go into migration or hibernation for the coming winter.
Today Samhain is observed by wearing costumes (as to distract the dark Sidhe spirits) and pumpkins are carved grotesquely for the same effect. Many build shrines do deceased loved ones and occasionally dumb/silent suppers are held to share a meal with the dead. The idea of Trick or Treating likely comes from those pesky Sidhe spirits who sometimes will give you gifts and otherwise will bring pain.
Samhain also marks the Wiccan New Year.