According to the Greek Orthodox tradition, nearly every day of the year is dedicated to a Christian saint or Martyr. When someone is named after one of those saints, that day becomes their "name day" and is celebrated.
The saints are divided into six main categories:
1) The Apostles
2) The Prophets
3) The Martyrs
4) The Fathers & Hierarchs
5) The Monastics
6) The Just.
All saints have the grace of God to help those who invoke them.