The first mention of All Saints Day dates from around the 4th century. Traditionally, All Saints was observed in the springtime, which it still is among Eastern Christians. As with many truly great things, Halloween was brought into being by the ancient Celts who lived in Ireland, Scotland, Wales and Brittany. The celebration preceded their conversion to Christianity. For the ancient Celts, 1 November marked the beginning of a new year and the coming of winter. The night before the new year, they celebrated the festival of Samhain, who, in Celtic mythology, was Lord of the Dead.
It was believed that during this festival the souls of the dead, including ghosts, goblins and witches, returned to mingle with the living. In order to scare away the evil spirits, people would wear masks and light bonfires. When the Romans conquered the Celts, showing a distinct lack of the requisite Celtic spirit, they added their own touches to the Samhain festival, such as making centerpieces out of apples and nuts for Pomona, the Roman goddess of the orchards. The Romans also bobbed for apples and drank cider! So, where, you might ask, does the Christian aspect of the holiday begin?
Well, in AD 835, Pope Gregory IV moved the celebration for all martyrs (later all saints) from 13 May to 1 November. The night before became known as All Hallow’s Eve or "holy evening." Eventually the name was shortened to the current Halloween.
Sadly, because All Saints falls on a Monday this year, it is not a holy day of obligation in the United States. However, it would be difficult for me to imagine not going to Mass on All Saints and All Souls.