It celebrates 7 days after Holy Easter
The Apostle Thomas was absent when Christ, after His Resurrection, visited His Disciples in the upper room where they were confined. When he was informed about the visit of Christ, he asked to see Him and feel the wounds of the Cross in His hands and side. When Christ visited His Disciples again after eight days, he invited the Apostle Thomas to feel the marks of wounds on His Body.
Then the Apostle Thomas recognized Him and confessed Him as his Lord and God. He recognized Him by the wounds of the Cross, which are a sign of His love, but also of His power. The Holy Fathers call the confession of Thomas salvation. And it truly leads to salvation all those who turn to Christ humbly seeking His mercy.
The fact that the Apostle Thomas was initially absent during the appearance of Christ to His Disciples, seems to have been an economy of God, so that the miracle of the Resurrection would be believed and all kinds of doubt would be dispelled.
The Apostle Thomas, after Pentecost, preached the Gospel to the Parthians, Persians, Medes and Indians and had a martyr’s end.
