The Synaxis of the Glorious and All-Praiseworthy Twelve Apostles of Christ appears to be an ancient Feast. The Church honors each of the Twelve Apostles on separate dates during the year, and has established a general commemoration for all of them on the day after the commemoration of the Glorious and First-Ranked among the Apostles, Peter and Paul. These are the names and the separate days of the celebrations of the Holy Twelve:
SAINT PETER June 29 and January 16
SAINT ANDREW November 30
SAINT JAMES, THE SON OF ZEBEDEE April 30
SAINT JOHN THE THEOLOGIAN September 26 and May 8
SAINT PHILIP November 14
SAINT BARTHOLOMEW June 11 and August 25
SAINT THOMAS October 6
SAINT MATTHEW THE EVANGELIST November 16
SAINT JAMES, THE SON OF ALPHAEUS October 9
SAINT THADDEUS OR JUDE, THE BROTHER OF JAMES June 19
SAINT SIMON THE ZEALOT May 10
SAINT MATTHIAS August 9
SAINT PAUL June 29
Let us also mention how each one of these most holy and most beneficial men in the history of the world ended their earthly life:
St. Peter was crucified upside down.
St. Andrew was crucified on an X-shaped cross.
Saint James, the son of Zebedee was beheaded.
Saint John the Theologian died in a miraculous manner.
St. Philip was crucified.
St. Bartholomew was crucified, scraped and beheaded.
St. Thomas was pierced with five spears.
Saint Matthew the Evangelist was burned alive.
Saint James, the son of Alphaeus was crucified.
Saint Thaddeus or Jude, the Brother of James was crucified.
Saint Simon the Zealot was crucified.
Saint Matthias was stoned and then was beheaded after death.
Saint Paul was beheaded.
The holy God-crowned Emperor Constantine the Great (May 21) built a church in Constantinople in honor of the Twelve Apostles. It was second only to Hagia Sophia among the great churches of Constantinople. Most Emperors and many patriarchs and bishops were buried in the church and their relics were venerated by the faithful for centuries. The most treasured possession of the church were the supposed skulls of Saints Andrew, Luke and Timothy, but the church also held relics of Saint John Chrysostom and other Church Fathers, saints and martyrs. The church also held what was believed to be part of the "Column of Flagellation", to which Jesus had been bound and flogged. The basilica was looted during the Fourth Crusade in 1204. The historian Nicetas Choniates records that the Crusaders plundered the imperial tombs and robbed them of gold and gems. Not even Justinian's tomb was spared. The tomb of Emperor Heraclius was opened and his golden crown was stolen along with the late Emperor's hairs still attached on it. Some of these treasures were taken to Venice, where they can still be seen in St Mark's Basilica. When Michael VIII Palaeologus recaptured the city from the Crusaders, he erected a statue of the Archangel Michael at the church to commemorate the event, and himself. In 1461, following the fall of Constantinople to Mehmed, it was taken over by the Ottomans and demolished to make way for the Fatih Mosque, which still occupies the site and houses Mehmed's tomb. Churches such as the Basilica of St. John, St Mark's Basilica, and Cathédrale Saint-Front are believed to have been modeled on Holy Apostles, but they differ from each other significantly. A possible reconstruction can be seen here. (Source)
For lists of the Apostles' names, see: Mt.10:2, Mark 3:14, Luke 6:12, Acts 1:13, 26.
Apolytikion in the Third Tone
O Holy Apostles, intercede to our merciful God, that He may grant our souls forgiveness of sins.
Kontakion in the Second Tone
O Lord, receive the steadfast and divinely voiced preachers, the pinnacle of Your disciples, unto their rest and the enjoyment of Your blessings. You received, above every offering, their labors and their life. You alone know what the heart holds.