All Saints' Day
All Saints' Day is a solemn holy day of the Catholic Church celebrated annually on November 1. The day is dedicated to the saints of the Church, that is, all those who have attained heaven. It should not be confused with All Souls' Day, which is observed on November 2, and is dedicated to those who have died and not yet reached heaven.
Although millions, or even billions of people may already be saints, All Saints' Day observances tend to focus on known saints --that is those recognized in the canon of the saints by the Catholic Church.
Other countries have different rules according to their national bishop's conferences. The bishops of each conference have the authority to amend the rules surrounding the obligation of the day.
All Saints' Day was formally started by Pope Boniface IV, who consecrated the Pantheon at Rome to the Virgin Mary and all the Martyrs on May 13 in 609 AD. Boniface IV also established All Souls' Day, which follows All Saints
Roman Catholic Obligation
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.