St. Constantine and Helena Day is celebrated on May 21st in honor of Saint Constantine and his mother, Saint Helena. This day commemorates their significant contributions to Christianity, particularly Constantine's role in the Edict of Milan, which granted religious tolerance in the Roman Empire. The holiday is observed in various countries, especially in Eastern Orthodox and Catholic traditions. Festivities often include church services, processions, and cultural events that highlight the legacy of these saints, who are revered for their faith and dedication to spreading Christianity throughout the empire.