History Lesson, Anyone?
In the Roman Catholic Church, All Saints’ Day is a Holy Day of Obligation in many (but not all) countries, meaning going to Mass on the date is required unless one has a good reason to be excused from that obligation, such as illness. However, in a number of countries that do list All Saints’ Day as a Holy Day of Obligation, including England & Wales, the solemnity of All Saints’ Day is transferred to the adjacent Sunday if 1 November falls on a Monday or a Saturday, while in the same circumstances in the United States the Solemnity is still celebrated on November 1 but the obligation to attend Mass is abrogated.
The origin of the festival of All Saints celebrated in the West dates to May 13, 609 or 610, when Pope Boniface IV consecrated the Pantheon at Rome to the Blessed Virgin and all the martyrs; the feast of the dedicatio Sanctae Mariae ad Martyres has been celebrated at Rome ever since. There is evidence that from the fifth through the seventh centuries there existed in certain places and at sporadic intervals a feast date 13 May to celebrate the holy martyrs. The origin of All Saints’ Day cannot be traced with certainty, and it has been observed on various days in different places. However, there are some who maintain the belief that it has origins in the pagan observation of 13 May, the Feast of the Lemures, in which the malevolent and restless spirits of the dead were propitiated. Liturgiologists base the idea that this Lemuria festival was the origin of that of All Saints on their identical dates and on the similar theme of “all the dead”.
The feast of All Saints, on its current date, is traced to the foundation by Pope Gregory III (731.741) of an oratory in St. Peter’s for the relics “of the holy apostles and of all saints, martyrs and confessors, of all the just made perfect who are at rest throughout the world”, with the day moved to 1 November and the 13 May feast suppressed.
Information found on catholic.org