St. Peter’s Chains ~ August 1st.
This feast commemorated the liberty of Peter, when he was freed from the prison of Herod by an angel (Acts 12:6). But two other events related to the relics of his chains are also a factor in why this feast was kept. The first was when Pope Alexander was imprisoned by the tribune Quirinus. Quirinus’s daughter was sick, and he promised to embrace the Christian faith if Pope Alexander could obtain a cure for her. When Quirinus brought his daughter to him, he told her to find the chains Peter was bound with (under Nero) and when she kissed them she should be healed. Quirinus found the chains, and his daughter was indeed cured.
For the second, the old Roman breviary tells us how a woman named Eudocia went to Jerusalem and was given the Peter’s chains from when he was bound by Herod. She venerated the chains, and sent them on to he daughter Eudoxia in Rome. The latter took the chains to the Pope who brought out the chains that Quirinus had found. When they were laid together they joined into one chain. Because of this and other miracles related to the chains, they began to be held in high esteem. At the time, August 1st was celebrated by pagans in honor of the Octavius, the first Emperor and the first to be called Augustus (from whom this moth gets its name). To overcome this pagan festival, the Pope rededicated a church on August 1st, calling it St. Peter Ad Vincula (at the chains) and he placed the miraculous chain therein. So the dedication of that church named in honor of the St. Peter’s chains is probably the main reason this feast was celebrated.
However another reason for the establishment of this, given in the Golden Legend, is this:
The Lord miraculously loosed Peter’s bonds and gave him the power to bind and to loose. We, on the other hand, are held and bound by the bonds of sin, and need to have those bonds loosed. Therefore we honor Peter on the feast called ad vincula, in order that as he merited to be freed of his bonds and received from the Lord the power of loosing, he may absolve us of the bonds of sin. That this really constituted a reason for the establishment of the feast is clear from the liturgical readings in the day’s Mass. The epistle commemorates the apostle’s liberation from his chains, and the gospel narrates the conferring upon him of the power of absolution, while the prayer [note: he is referring to the Collect] of the Mass asks that absolution be given to us through him.
That we all need help to be loosed from the bonds of sin is a lesson we could all use today. Not to sound like a broken record, but this another liturgical observance that was cut at a very poor time. Here is the Collect referred to by Jocubus de Voragine:
O God Who, breaking the chains that bound blessed Peter the Apostle, didst bid him to go forth unharmed : free us, we beseech Thee, from the bonds of sin, and in Thy great mercy keep far from us all evil. Through our Lord.
Source: BenedictusDeus blog which you can read here.