All Saints Day
Christians celebrate all their saints and martyrs on November 1. The tradition began in 4th century and is possibly related to the pagan tradition called Feast of the Lemurs that was held in honor of all the dead.
Throughout history numerous dates were designated for honoring different Christian saints. Eventually, due to a large number of saints November 1 was chosen for remembering all of them and showing gratitude for their help and appreciation for Jesus Christ.
For Roman Catholics tradition was started by Pope Bonface IV who, upon receiving the Pantheon at Rome from The Emperor Phocas dedicated it to all the martyrs. The celebration itself took place on May 13.
The Orthodox Church followed suggestion by St. Chrysoston of Constantinopole to honor the saints on the first Sunday after Pentecost which falls at the end of Easter season. Bishop of Rome, Gregory III moved the All Saints' Day to Novemebr 1, and around 835 Gregory IV officially ordered the holiday to be celebrated on this day by all Christians.