The word saint stems from the Greek word for "holy" or "set apart" (source). In Catholicism, saints are an esteemed group of believers who have been canonized (posthumously) for their good deeds on earth. In the Protestant tradition, the word saint is often used to refer to any believer, as it is throughout the New Testament (source).
"And He Himself gave some to be apostles, some prophets, some evangelists, and some pastors and teachers, for the equipping of the saints for the work of ministry, for the edifying of the body of Christ." - Ephesians 4:11-12
All Saints' Day is November 1 each year, but it is celebrated in the church on the following Sunday. Some congregations honor the memory of those lost in the past year; others seek ways to honor and celebrate the work of God's saints in the church, in the community, and the world today.