All Saints' Day (also known as All Hallows' Day or Hallowmas) is the day after All Hallows' Eve (Hallowe'en). It is a feast day celebrated on 1st November by Anglicans and Roman Catholics.
It is an opportunity for believers to remember all saints and martyrs, known and unknown, throughout Christian history. As part of this day of obligation, believers are required to attend church and try not to do any servile work.
Remembering saints and martyrs and dedicating a specific day to them each year has been a Christian tradition since the 4th century AD, but it wasn't until 609AD that Pope Boniface IV decided to remember all martyrs. Originally 13th May was designated as the Feast of All Holy Martyrs. Later, in 837AD, Pope Gregory IV extended the festival to remember all the saints, changed its name to Feast of All Saints and changed the date to 1st November.