Yesterday was All Saints' Day. In the Western tradition, it's a time to honor Christian saints and martyrs. Pope Boniface IV originated the holy day in around 609, in the process of consecrating the Pantheon of Rome to the Virgin Mary. At that time, All Saints' Day was celebrated on May 13. Pope Gregory III changed it to November 1 in the mid-700s, possibly to coincide with, and incorporate, the Celtic pagan festival of Samhain, a time when the border between the dead and the living was especially porous and ghosts were believed to walk among the living.
All Saints' Day is a national holiday in some countries, and it's followed by the Feast of All Souls on November 2, which honors the non-saintly Christian dead. In Mexico and Central America, it marks the first day of the Día de los muertos, or Day of the Dead, festival. Day of the Dead traditions include building altars at the graves of deceased family members and leaving offerings of gifts, favorite foods, marigolds, and elaborately decorated sugar skulls.