On All Hallows' Eve, Catholics were asked to follow the Church's tradition of observing a fast day before a major feast.
For many children in the United States, Halloween is their favorite holiday of the year. This is primarily the case because of all the candy children get to feast on after going trick-or-treating.
Interestingly, while this secular holiday has become associated with feasting on candy, it was originally a day of fasting in the Catholic Church.
November 1 is All Saints Day in the Catholic Church, and is usually a holy day of obligation. (There is no obligation this year for Catholics in the US, as it falls on a Monday.) All Saints Day is a solemnity, one of the highest ranked feasts in the Church, a day marked with joy and celebration.
For many centuries the Church was accustomed to observe the eve or vigil of a major feast day with a special fast. It followed the ancient saying that you should “fast before you feast.”
The Catholic Encyclopedia explains a bit of the history behind this tradition.
In the first ages, during the night before every feast, a vigil was kept. In the evening the faithful assembled in the place or church where the feast was to be celebrated and prepared themselves by prayers, readings from Holy Writ (now the Offices of Vespers and Matins), and sometimes also by hearing a sermon … In place of nocturnal observances, the bishops introduced for the laity a fast on the day before the feast … The Synod of Seligenstadt (1022) mentions vigils on the eves of Christmas, Epiphany, the feast of the Apostles, the Assumption of Mary, St. Laurence, and All Saints.
The day prior to All Saints (October 31) was observed as a day of fasting for many centuries, all the way through the first half of the 20th century, as is mentioned in the Catholic Encyclopedia of that era. “In the United States only four of theses vigils are fast days: the vigils of Christmas, Pentecost, the Assumption, and All Saints.”
It may seem strange to modern ears that Halloween was a day of fasting, but Christians in previous centuries saw this day as a day to prepare their hearts for the celebration of All Saints.