Each year on the Orthodox calendar June 30 is dedicated to the Synaxis of the Twelve Apostles. Each of the Apostles has their own individual feast day on the calendar, this one celebrates the Twelve as a community. This feast may be related to the establishment of a church in Constantinople in the 4th Century which was dedicated to the Twelve.
“The Twelve evidently constituted the earliest Christian ‘canon’ or measuring-rod – the standard by which the authenticity of the Church’s message was to be gauged, for the duration of their lifetime. They are the pillars of the whole structure (cf., though with wider reference, Gal. 2:9, 1 Tim. 3:15), and to such one must refer one’s preaching (Gal. 2:2). But there is no sign that the Twelve were intended to be perpetuated by succession. here was no caliphate: if there was a caliphate anywhere in the Christian Church, it was in the line of James the Lord’s brother, not of the Twelve. They were regarded as essentially a dominically chosen and commissioned foundation body, expressly authorized to give eye-witness evidence of the decisive events. As such they were, by definition, irreplaceable in any subsequent generation. Whereas the removal of Judas by apostasy was met by the special lot-casting for Matthias, none of the subsequent depletions, by martyrdom or natural death, were made up. The Twelve were no self-perpetuating body: they were simply the initial authority for the Christian claims about Jesus. Alongside this authority, and indeed as an integral part of it, there ran also the authority of the Jewish scriptures. The ‘argument from Scripture’, that is, the demonstration that what the apostles bore witness to was no isolated phenomenon, but could be shown to be the culmination and fulfillment of God’s design for his People already sketched in scripture.” (C.F.D. Moule, The Birth of the New Testament, pgs. 179-180)