The Exaltation of the Holy Cross, when Emperor Heraclius, after defeating King Chosroes, brought it back to Jerusalem from Persia. The lesson below is taken from today's Office of Matins from the Roman Breviary:
Chosroës, King of Persia, having, in the last days of the reign of the Emperor Phocas, overrun Egypt and Africa, took Jerusalem, where he slaughtered thousands of Christians and carried off to Persia the Cross of the Lord, which Helen had put upon Mount Calvary. Heraclius, the successor of Phocas, moved by the thought of the hardships and horrid outrages of war, sought for peace, but Chosroës, drunken with conquest, would not allow of it even upon unfair terms. Heraclius therefore, being set in this uttermost strait, earnestly sought help from God by constant fasting and prayer, and through his good inspiration gathered an army, joined battle with the enemy, and prevailed against three of Chosroës his chief captains, and three armies. Chosroës was broken by these defeats, and when in his flight, he was about crossing the Tigris, he proclaimed his son Medarses partner in his kingdom. Chosroës' eldest son Siroës took this slight to heart, and formed a plot to murder his father and brother, which plot he brought to effect soon after they had come home. Then he got the kingdom from Heraclius upon certain terms, whereof the first was that he should give back the Cross of the Lord Christ.
The Cross therefore was received back after that it had been fourteen years in the power of the Persians, and Heraclius came to Jerusalem and bore it with solemn pomp unto the Mount whereunto the Saviour had borne it. This event was marked by a famous miracle. Heraclius, who was adorned with gold and jewels, stayed perforce at the gateway which leadeth unto Mount Calvary, and the harder he strove to go forward, the harder he seemed to be held back, whereat both himself and all they that stood by were sore amazed. Then spake Zacharias, Patriarch of Jerusalem, saying: See, O Emperor, that it be not that in carrying the Cross attired in the guise of a Conqueror thou shewest too little of the poverty and lowliness of Jesus Christ. Then Heraclius cast away his princely raiment and took off his shoes from his feet, and in the garb of a countryman easily finished his journey, and set up the Cross once more in the same place upon Calvary whence the Persians had carried it away. That the Cross had been put by Heraclius in the same place wherein it had first been planted by the Saviour caused the yearly Feast of the Exaltation of the Holy Cross to become the more famous thenceforward.