All Saints' Day - or Solemnity of All Saints - is a solemnity celebrated on the first of November in honor of all the saints, either known and unknown. It is the day after the feast of All Hallows' Evening (also known as Halloween). This solemnity comes from the Christian tradition of celebrating the martyrdom of saints on the anniversary of their death.
In the early days the feast of All Saints was celebrated on the 13 May but in 731 Pope Gregory III moved it to 1 November. For many Catholic countries it is a national holiday.