The Memorial of the Korean Martyrs
September 20 is the feast day for the 103 martyrs of Korea during the persecution which lasted from around 1839 to 1867.
Christianity came to Korea around 1592, likely when it was introduced by an invading Japanese force. At the time though, it was introduced by soldiers who left when the troops withdrew. But unknown to the rest of the world, it actually continued to survive there until 1839 when the first priest visited there from China and was amazed to find a small community of about 4 thousand Catholics, though they had never seen a priest. The small community had continued to pray together, baptize, and celebrate marriages while longing to someday have access to the other sacraments. After the arrival of priests, the faith exploded and blossomed to over 10 thousand Catholics in just a few years. This likely made the local authorities quite nervous, leading to the persecution.
Up to around 1839, the priests in Korea were foreign missionaries. At that point, a request was sent by layman Paul Chong Hasang to Rome asking Pope Gregory XVI to create a diocese in Korea. Paul would eventually be martyred by crucifixion, but the Holy Father complied and sent a French bishop to head the infant diocese. The bishop quickly set about nourishing the new flock and St. Andrew came forward, as the first candidate for Holy Orders. Young Andrew traveled to China to attend seminary and was eventually able to sneak back into Korea only to be beheaded a short time later at the age of 25. So influential was St. Andrew, that on his death bed, the French bishop asked to be buried next to St. Andrew whom he loved as a son. He is patron saint of Korea and his blood and that of his companions, had been the seed for the Current Church there hosting more than five million Catholics today.