The Celts believed that it was a time when the veil between the world of the living and that of the dead was very thin. Sometimes they honored those who were dead by preparing a feast and setting a place for the deceased person. In this way, it is similar to the Latin American festival Dia de los Muertos, or the Christian holiday All Souls Day.
In addition to honoring the dead, the Celts gave offerings to fairy mounds as a form of protection to ward off any spells during this time. They believed that this night could be perilous because of harmful spirits and used small lanterns made of turnips (sometimes with grinning faces cut into them) to ward off evil (see below). This is the origin of our jack-o-lantern.
So, however you celebrate this night, I wish you well.