In many Christian denominations, today is All Saints' Day, although in Eastern Christianity, the festival is the first Sunday after Pentecost. It is the middle day of Hallowmas, the three-day festival commemorating those saints, known and unknown, who have died (in Catholic theology: those who have attained the beatific vision in Heaven). Many churches have a recitation of the names of members of that congregation who have died during the previous year. As this site indicates, the feast was mentioned in a sermon as early as 373 AD, and the date of November 1 was instituted by the 8th century Pope Gregory III, while the 9th century Pope Gregory IV made it a feast of the entire church. See also this site for the meaning of being a saint.